Radio access networks

ABSTRACT

Among other things, a communication system comprising remote units and a controller is described. The remote units wirelessly exchanges radio frequency (RF) signals with mobile devices. Each RF signal comprises information destined for, or originating from, at least one of the mobile devices. The remote units and the controller communicate baseband data corresponding to the information across an intermediate network. The remote units each implement at least some physical layer processing for an air interface used to wirelessly communicate with the mobile devices. Two or more of the remote units transmit a downlink reference signal to a mobile device of the mobile devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.17/374,666, filed on Jul. 13, 2021 and titled “RADIO ACCESS NETWORKS,”which is a continuation of U.S. application Ser. No. 16/152,266, filedon Oct. 4, 2018 and titled “RADIO ACCESS NETWORKS” (issued on Aug. 24,2021 as U.S. Pat. No. 11,102,663) which is a continuation of U.S.application Ser. No. 15/191,005, filed on Jun. 23, 2016 and titled“RADIO ACCESS NETWORKS” (issued on Nov. 27, 2018 as U.S. Pat. No.10,142,858), which is a continuation of U.S. application Ser. No.13/762,284, filed on Feb. 7, 2013 and titled “RADIO ACCESS NETWORKS”(issued on Jun. 28, 2016 as U.S. Pat. No. 9,380,466), the contents ofall of which are hereby incorporated by reference.

TECHNICAL FIELD

This disclosure relates to radio access networks.

BACKGROUND

The widespread use of mobile devices, such as smartphones, has increasedthe demand for mobile data transmission capacity and for consistent andhigh-quality radio frequency (RF) coverage at in-building and otherdensely populated locations. Traditionally, inside buildings, mobileoperators rely on a Distributed Antenna System (DAS) to allow users toconnect to the operators' networks for voice and data transmission.

SUMMARY

In one aspect, this disclosure features a communication systemcomprising remote units and a controller. Each of the remote unitscomprises one or more radio frequency (RF) units to exchange RF signalswith mobile devices. At least some of the RF signals compriseinformation destined for, or originating from, a mobile device. Thecontroller comprises one or more modems and is connected to an externalnetwork. At least one of the modems is a baseband modem and isconfigured to pass first data corresponding to the information. The atleast one of the modems is configured to perform real-time scheduling ofthe first data corresponding to the information. The controller isseparated from the remote units by an intermediate network. Theintermediate network comprises a switched Ethernet network over whichsecond data corresponding to the information is carried in framesbetween the controller and the remote units.

In another aspect, this disclosure features a communication systemcomprising remote units, a reference timing source, a controller, acontroller clock, and a remote unit clock. The remote units exchangeradio frequency (RF) signals with mobile devices. At least some of theRF signals comprise information destined for, or originating from, amobile device. The reference timing source is synchronized with acoordinated universal time (UTC) or a Global Positioning System (GPS).The controller comprises one or more modems and is connected to anexternal network. At least one of the modems is a baseband modem and isconfigured to pass first data corresponding to the information. Thecontroller is separated from the remote units by an intermediate networkover which second data corresponding to the information is transmittedin frames between the controller and the remote units. The second datacomprises baseband data. The controller clock is synchronized with thereference timing source. The controller clock provides timinginformation to the controller. The remote unit clock is synchronizedwith the controller clock. The remote unit clock provides timinginformation to a remote unit. The controller and the remote unit areconfigured to transmit time stamp messages to synchronize the controllerclock and the remote unit clock. The controller and the remote units areconfigured to transmit the time stamp messages by avoiding contentionbetween time stamp transmissions and baseband data transmissions orbetween time stamp transmissions of different remote units to thecontroller.

In another aspect, the disclosure features a communication systemcomprising remote units and a controller. The remote units exchangeradio frequency (RF) signals with mobile devices. At least some of theRF signals comprise information destined for, or originating from, amobile device. The controller comprises one or more modems and isconnected to an external network. At least one of the modems is abaseband modem and is configured to pass first data corresponding to theinformation. The controller is separated from the remote units by anintermediate network over which second data corresponding to theinformation is carried in frames between the controller and the remoteunits. The second data comprises baseband data and the intermediatenetwork is configured to transport in frames baseband data. At leastsome of the baseband data is compressed in a frequency domain. Theremote units and the controller are configured to compress the basebanddata for transmission over the intermediate network.

The aspects of the disclosure may also include one or more of thefollowing features. The intermediate network comprises multipleswitches. The external network comprises the Internet. The mobiledevices are cellular communication devices that communicate using thelong term evolution (LTE) standard. The remote units are configured toperform some modem functionality. The controller is devoid of RF radiofunctionality. The switched Ethernet network comprises multipleswitches. At least one of the multiple switches is connected to at leastone remote unit over a 1 gigabit/second Ethernet link. Each remote unitcomprises multiple RF antennas and is configured to transmit and/orreceive RF signals from one or more mobile devices simultaneously overone or more radio channels. The controller comprises one or moreprocessing devices, the one or more processing devices being programmedto associate one or more of the modems with one or more of the remoteunits to thereby configure communication cells that comprise one or moreremote units. The one or more processing devices are programmed toassociate one or more of the modems with one or more of the remote unitsto thereby configure the communication cells dynamically. The one ormore modems control a set of the remote units through the switchedEthernet network to form a cell, each remote unit in the cell comprisingone or more antennas, the one or more antennas being associated with acommon cell identifier. The common cell identifier comprises the longterm evolution (LTE) Cell-ID. All remote units associated with the cellare configured to communicate over a single long term evolution (LTE)channel. Each remote unit associated with the cell comprises a pair ofantennas, and at least two pairs of antennas of remote units associatedwith the cell are controllable to communicate with a single pair ofantennas on a single mobile device. Each remote unit associated with thecell comprises one or more antennas. Each antenna corresponds to avirtual antenna port. All antennas assigned to a same virtual antennaport simulcast a common signal. The remote units assigned to the samevirtual antenna port carry the same LTE downlink reference signalsassociated with the same virtual antenna port. The virtual antenna portincludes a Channel State Information Reference Signal (CSI-RS)scrambling ID. The mobile device sends more than one Channel StateInformation (CSI) feedback. Each of the antennas of the remote units isassigned to a different virtual antenna port. The remote units in thecell are synchronized to communicate using a same frequency. The remoteunits in the cell are configured to implement a network-basedsynchronization protocol to effect synchronization. The controllercomprises one or more processing devices, the one or more processingdevices being programmed to modify an association of one or more of themodems with one or more of the remote units to thereby re-configureexisting communication cells defined by one or more remote units.Re-configuring existing communication cells comprises splitting at leastone existing communication cell into two or more new communicationcells. Re-configuring existing communication cells comprises combiningat least two existing communication cells into a single newcommunication cell. The controller is configured to modify theassociation based on commands received from a management system. Thecontroller is configured to modify the association based on time-of-day.The controller is configured to modify the association based on changesin a distribution of demand for communication capacity. The cell isconfigured to virtually split to send data to two or more mobile deviceson the same resources without substantial interference based on radiofrequency isolation between the two or more mobile devices. Theresources are time-frequency resources of long term evolution (LTE). Thecontroller is configured to determine which mobile devices to send dataon the same resource based on signals received from the mobile devices.The mobile devices comprise receivers and the data sent to the receiversby the remote units in the cell is not on the time-frequency resource.The cell is configured to virtually split to receive information fromtwo or more mobile devices on the same resources without substantialinterference based on radio frequency isolation between the two or moremobile devices. Two or more mobile devices use the same demodulationreference sequence. The two or more mobile devices use the same PUCCHresource consisting of a cyclic shift and orthogonal cover code. Thecontroller is configured to detect RACH preamble transmissions from thetwo or more mobile devices sent in the same PRACH opportunity. Thecontroller comprises one or more processing devices, the one or moreprocessing devices being programmed to associate one or more additionalmodems with one or more of the remote units in response to a change indemand for communication capacity. In response to a decrease in demandfor network capacity, the one or more processing devices are programmedto consolidate the one or more remote units among a decreased number ofthe one or more modems. The cell is a first cell and the modem is afirst modem; and the one or more modems comprise a second modemprogrammed to control a second set of the remote units through theswitched Ethernet network to form a second cell, each RF unit in thesecond cell comprising one or more second antennas, the one or moresecond antennas being associated with a second common cell identifier.The first cell and the second cell comprise different numbers of remoteunits, different shapes, and/or transmit radio signals coveringdifferent sized areas. The controller comprises one or more processingdevices, the one or more processing devices being programmed toassociate the first and second modems with different remote units inorder to dynamically change shape and/or an area covered by each of thefirst cell or the second cell. The first and second modems areco-located with the controller, and the controller coordinates thetransmissions of the first and second modems to reduce interferencebetween the first and second cells. At least one remote unit isconfigured to exchange Wi-Fi signals with a corresponding device. Thecontroller comprises one or more processing devices, the one or moreprocessing devices being programmed to receive second data from theswitched Ethernet network and to process the second data to generatefirst data. At least some of the remote units are configured to receivepower through the switched Ethernet network. The controller and theremote units are configured to communicate using the IEEE1588 protocol.The communication system also includes a network manager incommunication with the controller that directs operation of thecontroller. The external network comprises an operator's core networkand the network manager is located in the operator's core network. Thenetwork manager is located locally with respect to the controller. Twoor more remote units are configured to send the second data to a mobiledevice on two or more RF channels so that the mobile receives the seconddata simultaneously from the two or more remote units. The controller isconfigured to aggregate communication from different channels betweenthe controller and the remote units and the controller and the externalnetwork to process the first data and to send the second data to theremote units.

The aspects of the disclosure may also include one or more of thefollowing features. The first data comprises Internet Protocol (IP) dataand the controller is configured to perform real-time media accesscontrol of the IP data corresponding to the information. The referencetiming source comprises a GPS receiver. The GPS receiver is located inthe controller. The controller and the remote units are configured toexchange time stamps using the IEEE 1588 protocol. The controller andthe remote units comprise a system-on-chip to generate and process thetime stamp messages. The intermediate network is a switched Ethernetnetwork. The remote unit uses the time stamp messages to estimate andcorrect an error of the remote unit clock. The estimation is based on apriori knowledge about downlink and uplink time stamp delays. The apriori knowledge about the downlink and uplink time stamp delayscomprises a ratio of the downlink time stamp delay to the uplink timestamp delay. The a priori knowledge about the downlink and uplink timestamp delays comprises a ratio of an average downlink time stamp delayto an average uplink time stamp delay. The error comprises a timingphase error and the remote unit is configured to estimate the timingphase error by weighting and/or offsetting measured time stamps in theuplink and the downlink according to the a priori knowledge. The timestamp messages are transmitted with high priority according to the IEEE802.1q protocol. The time stamp messages and the baseband data aretransmitted on different virtual local area networks (VLANs). The timestamp messages and the baseband data are transmitted on the same virtuallocal area network (VLAN) using different priority markings of the IEEE802.1q protocol. The baseband data and the time stamp messages aretransmitted using dedicated Ethernet ports and dedicated Ethernet linksof the switched Ethernet network. The communication system comprises aplurality of controllers and one of the controllers is a mastercontroller and is configured to transmit the time stamp messages withremote units associated with the master controller and with remote unitsassociated with the other controllers of the plurality of controllers.The controller is configured to advance in time a subframe of basebanddata to be delivered to a remote unit to compensate a time delay betweenthe remote unit clock and the controller clock. The controller isconfigured to advance in time the subframe of baseband data for apre-determined amount. The pre-determined amount is determined based ona time delay for transmitting the baseband data over the intermediatenetwork. The controller is configured to send information to the mobiledevices for the mobile devices to advance a timing phase of the RFsignals to be transmitted to the remote units relative to the RF signalsreceived by the mobile devices from the remote units. The controller isconfigured to increase processing time available to the controller forthe controller to process the baseband data transmissions by choosing anamount of the timing phase to be advanced to be greater than a timedelay for transmitting RF signals in a round trip between a remote unitand a mobile device. A remote unit is configured to advance in timesubframes of the baseband data to be transmitted to the controller. Theremote units are configured to communicate with the controller on acommunication channel, and a frequency of the communication channel isderived from the controller clock. The controller clock comprises acrystal oscillator configured to generate clocks for baseband processingin the controller. The remote unit clock comprises a crystal oscillatorconfigured to generate clocks for analog-digital-analog converters(A/D/As), RF synthesizers, and/or baseband processing in each remoteunit. The controller and the remote unit are configured to transmit timestamp messages in multiple round-trips between the controller and theremote unit. The remote unit is configured to adjust the remote unitclock based on one of the transmissions in multiple round-trips that isdeemed to be most reliable to correct an offset between the controllerclock and the remote unit clock. The one of the transmissions inmultiple round-trips that is deemed to be most reliable comprises atransmission that predicts a smallest offset between the controllerclock and the remote unit clock. The remote unit is configured to not tomake any correction to the remote unit clock when an estimate of anoffset between the controller clock and the remote unit clock based onthe transmissions of the time stamp messages is deemed to be unreliable.The estimate of the offset is deemed to be unreliable when the estimateexceeds a pre-configured threshold. The controller clock is in directcoupling with the reference timing source and the remote unit clock isnot in direct coupling with the reference timing source.

The aspects of the disclosure may also include one or more of thefollowing features. A rate of transmission of the baseband data over theintermediate network is at most 1 Gb/s. The baseband data is representedby complex-valued signals having real and imaginary components, and thecontroller is configured to compress the baseband data by quantizing thecomplex-valued signals in the frequency domain to produce quantizedbaseband data, and to transmit binary data representative of thequantized baseband data to the remote units. The remote units areconfigured to reconstruct the quantized baseband data upon receipt ofthe compressed baseband data. The remote units are configured to applyan inverse fast Fourier transform on the reconstructed baseband data.The controller is configured to quantize the baseband data in thefrequency domain using a quantizer having a fixed rate and a fixed stepsize. The controller is configured to quantize independently the realand imaginary components of the baseband data in the frequency domain.The controller is configured to send information about the fixed rateand the fixed step size to the remote units when the remote units andthe controller are connected. The controller is configured to quantizethe baseband data in the frequency domain using a quantizer having afixed rate and an adjustable step size. The controller is configured tosend side information about the fixed rate and a step size to a remoteunit once per subframe. The controller is configured to quantize thebaseband data in the frequency domain using a quantizer having a rateand a step size. The rate and the step size both are adjustable. Thecontroller adjusts the step size according to energy of the quantizedbaseband data. The controller adjusts the rate according to a modulationand coding scheme of the baseband data. The RF signals are compatiblewith the long term evolution (LTE) standard. The controller isconfigured to send side information about the rate of the quantizer to aremote unit for each of plural resource element groups (REG) andphysical resource blocks (PRB) in each orthogonal frequency-divisionmultiplexing (OFDM) symbol of a subframe. The controller is configuredto compress the baseband data by not sending to the remote units anydata for unused resource element groups (REGs) or physical resourceblocks (PRBs) in each orthogonal frequency-division multiplexing (OFDM)symbol of the baseband data. The baseband data in the frequency domainbelongs to, or is derived from, a discrete-amplitude signalconstellation, and the controller is configured to compress the basebanddata without quantization by sending binary data representing thediscrete-amplitude signals to the remote units. The discrete-amplitudesignal constellation comprises a quadrature amplitude modulation (QAM)signal constellation. The RF signals carry orthogonal frequency-divisionmultiplexing (OFDM) symbols, and the controller is configured to sendthe binary data to the remote units in the same order as thecorresponding OFDM symbols are to be transmitted by the remote unitsover the air to the mobile devices. The remote units are configured tocompress the baseband data by quantizing the baseband data in thefrequency domain to produce quantized baseband data, and to transmitbinary data representative of the quantized baseband data to thecontroller. A remote unit is configured to receive data in time domainfrom the mobile device and to apply a fast Fourier transform to the datain the time domain to produce the baseband data in the frequency domain.A remote unit is configured to quantize the baseband data in thefrequency domain using a quantizer having a fixed rate and a fixed stepsize. A remote unit is configured to quantize the baseband data in thefrequency domain using a quantizer having a fixed rate and an adjustablestep size. The frames of the baseband data comprise orthogonalfrequency-division multiplexing (OFDM) symbols and the remote unit isconfigured to select a step size based on an average energy of thequantized baseband data. The average energy is an average of energies ofbaseband data that belong to a long term evolution (LTE) channel. Theremote unit is configured to select a step size based on a distributionof the baseband data in the frequency domain. The remote unit isconfigured to send side information about the quantizer to thecontroller for the controller to reconstruct the received quantizedbaseband data. A remote unit is configured to quantize the baseband datain the frequency domain using a quantizer having a rate and a step size,the rate and the step size both being adjustable. The frames of thebaseband data comprise subframes comprising LTE physical resource blocks(PRBs), and the remote unit is configured to adjust the rate of thequantizer on a per PRB basis. The remote unit is configured to select aquantizer rate based on a modulation and coding scheme of the basebanddata determined by the controller. The remote units are configured toquantize the baseband data using quantizers having adjustable rates. Thequantizer rates for the baseband data are adjusted according to the LTEresource blocks. The quantizer rates are chosen to be zero to purgetransmissions of the baseband data for some of the resource blocks. Thecontroller is configured to send side information to the remote unitsand the information is used by the remote units to determine thequantizer rates. The controller is configured to determine the sideinformation to be sent to the remote units based on information receivedfrom the mobile devices. The controller is configured to determine theside information based on a target signal-to-noise plus interferenceratio (SINR) at the controller. The information received from the mobiledevices corresponds to LTE Sounding Reference Signal (SRS) transmissionsby the mobile devices. The information received from the mobile devicescorresponds to LTE Physical Random Access Channel (PRACH) transmissionsby the mobile devices. The information received from the mobile devicescorresponds to uplink transmission on the Physical Uplink Shared Channel(PUSCH) by the mobile devices. A remote unit comprises two or morereceiver antennas for receiving the RF signals from the mobile devices,and the remote unit is configured to quantize the baseband datacorresponding to the different antennas using different quantizers. Thequantizers for different antennas have different step sizes. Thequantizers for different antennas have different step sizes anddifferent rates. The different rates are determined by the controller.The controller is configured to send side information to the remote unitto indicate the determined quantizer rate for each receive antenna. Aremote unit comprises two or more receiver antennas for receiving the RFsignals from the mobile devices. The remote unit is configured toquantize the baseband data using a quantizer having a rate selectedbased on correlation of the RF signals received at different receiversof the remote unit. The controller is configured to determine acoefficient based on the correlation of the RF signals and to determinethe rate of the quantizer using the coefficient. The remote unit isconfigured to determine the rate of the quantizer using a coefficientdetermined by the controller based on the correlation of the RF signals.The remote unit is configured to determine a coefficient based on thecorrelation of the RF signals and to determine the rate of the quantizerusing the coefficient. All baseband data except for those correspondingto Physical Random Access Channel (PRACH) transmissions from a mobiledevice is compressed in the frequency domain. A remote unit isconfigured to compress the baseband data by quantizing the receivedPRACH transmissions after performing a correlation in the frequencydomain. The remote unit is configured to compress the baseband data byquantizing the received PRACH transmissions in a time-domain afterconverting an output of the correlation back into the time domain. Atleast one modem of the controller is configured to execute real-timemedia access control (MAC) functions for the IP data corresponding tothe information.

Other features, objects, and advantages of the disclosure will beapparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of an example of a radio network.

FIGS. 2A and 2B are block diagrams showing an example of one cellconnected to a controller (CU) and two cells connected to a controller(CU).

FIG. 2C is a schematic diagram of an example of a remote unit (RU).

FIG. 3 is a block diagram schematically showing the deployment of anexample radio network on a site.

FIGS. 4A-4C are schematic block diagrams of examples of antenna mappingschemes in a cell.

FIG. 5A is a block diagram schematically showing an example of virtualsplitting in a cell.

FIG. 5B is a block diagram schematically showing an example of acontroller detecting Physical Random Access Channel (PRACH)transmissions.

FIGS. 6A and 6B are schematic block diagrams of an example of a radionetwork with different cell configurations at different times.

FIG. 7 is a block diagram showing examples of two resources grids fortwo respective antennas of a remote unit (RU).

FIG. 8 is a block diagram showing an example of signal transmissionsbetween a user equipment (UE) and a remote unit (RU).

FIG. 9 is a block diagram schematically showing an example of uplinkcompression.

FIG. 10 is a block diagram schematically showing an example of sideinformation on the uplink and the downlink between a controller (CU) anda remote unit (RU).

FIG. 11 is a block diagram schematically showing an example ofpredictive quantization for PUSCH.

FIG. 12 is a diagram showing an example of subframes boundaries.

FIG. 13 is a diagram showing an example of downlink hybrid automaticrepeat request (HARQ) operation.

FIG. 14 is a diagram showing an example of subframe alignment.

FIGS. 15 and 16 are diagrams showing examples of HARQ timing for thedownlink and the uplink, respectively.

FIG. 17A is a block diagram showing an example of Soft Frequency Reuse(SFR) in LTE.

FIG. 17B is a block diagram showing an example of two cells implementingcoordinated scheduling.

FIG. 18 is a flow diagram showing an example of synchronization betweena controller and a remote unit.

FIG. 19 is a schematic diagram showing an example of a special subframeused in transitioning from DL transmission to UL transmission.

FIG. 20A-20C are schematic diagrams showing examples of combiningsignals from different baseband modems at the controller, by thebaseband modems, and at the remote units, respectively.

DETAILED DESCRIPTION

Referring to FIG. 1 , a radio network 12 is deployed on a site 10 sothat one or more mobile operators, such as operator A 14, operator B 16,can provide mobile network access to one or more user equipments (UE(s))18, 20, such as smartphones, at the site 10. The site may be anenterprise or corporate building, a public venue, such as a hotel,hospital, university campus, or even an outdoor area such as a ski area,a stadium or a densely-populated downtown area. The radio network 12includes controllers (each of which can also be referred as a ControllerUnit (CU)) 22, 24 and Remote Units (RU) 26 a-26 i connected by anEthernet network 28. The CUs 22, 24 are connected (backhauled) to theoperator's core network, which may include nodes defined in the LongTerm Evolution (LTE) standard such as the mobility management entity(MME) 14 a, 16 a and Serving Gateways (SGW) 14 b, 16 b, optionallythrough Home eNodeB gateways (HeNB GW) 30, 32. The CUs may connect tothe operator's core network via the Internet or other IP-based packettransport network 33 (for the purpose of discussion, we may only referto the network 33 as the Internet, although other networks arepossible). The CUs may also include certain MME functionality (notshown) and SGW functionality (not shown), thus allowing traffic to flowdirectly between the UE and a destination node 31 on the Internet or onthe local IP network at the site 10 without traversing the operator'score network.

Each CU 22, 24 performs the functions of a base station, except forcertain baseband modem and RF functions that are performed by the RUs.Each CU also manages one or more of the RUs. Each CU may be associatedwith a mobile operator such that the RUs they manage may operate on aspectrum that belongs to that mobile operator. It is also possible for aCU to be shared between multiple mobile operators. Among other things,the CUs will schedule traffic to/from the UEs. Each CU 22, 24 is alsoconnected to a service manager 40, 42, which is typically located inoperator's core network. The service manager is responsible for theconfiguration, activation and monitoring of the radio network. There mayalso be a local facility service manager, which can allow a local ITpersonnel to install and maintain the radio network. The RUs 26 a-26 icontain the RF transceivers to transmit RF signals to and from the userequipment and perform RF front-end functions, among other functions.

Generally, a traditional base station, such as a traditional small cell,includes a Radio Frequency (RF) unit, a digital baseband modem unit anda network processing unit. Such a traditional base station performs boththe RF functionality and the baseband processing. In someimplementations, one or more traditional base stations can be incommunication with a centralized controller. The basebandfunctionalities can be split between the traditional base station andthe centralized controller of the traditional base station(s) such thatthe centralized controller performs only the upper layer (e.g., layer 3or higher) processing functions of the baseband functionality.

The CUs of the disclosure do not perform any RF functions. Each CU caninclude one or more baseband modems each for performing functions of alllayers of baseband functionalities, including the Media Access Control(MAC) layer (Layer 2) processing, and upper layer (Layer 3 and above)processing. For example, real-time scheduling, which is part of the MAClayer is performed by a baseband modem of a CU of the disclosure.Baseband modems may also perform physical layer (Layer 1) processing. Inaddition, the baseband modems or the CUs may also perform otherfunctions similar to the traditional base station, such as the functionof the network processing unit, e.g., processing Internet Protocol (IP)data.

In some implementations, real-time scheduling refers to assigning userdata to time and/or frequency resources based on CSI. In downlinkscheduling, CSI is supplied by the UE. In the LTE standard, the downlinkCSI may include a Channel Quality Indicator (CQI), Precoding MatrixIndicator (PMI) or Rank Indicator (RI). In uplink scheduling, CSI isdetermined by the controller based on transmissions received from theUEs. In the LTE standard, uplink CSI may be determined based on thesignals transmitted by the UE, for example the Sounding Reference Signal(SRS). The baseband modem functions performed by the controller may alsoinclude downlink error control coding, uplink error control decoding,uplink multi-antenna diversity combining of signals received bydifferent RUs, channel estimation, and other upper layer functionsrelated to the wireless transmission or reception.

The CUs and the RUs of the network 12 perform distinctive functions inthe radio network and are connected by the Ethernet network 28. The CUs22, 24 determine the capacity of the data/signal transmission at thesite 10, while the RUs 26 a-26 i provide RF/signal coverage to the site10.

The CUs 22, 24 contain one or more processors on which software isstored to instruct the processors to perform certain network andbaseband modem functions. The processors can be hardware formed byIntegrated Circuits (ICs) and other electrical components. Each CU 22,24 contains one or more baseband modem processors (see FIGS. 2A and 2B)or is configured to perform the functions of one or more basebandmodems. Each baseband modem may be implemented on one or multipleprocessors. When a baseband modem is implemented on multiple processors,each processor may be responsible for processing the signals associatedwith selected groups of UEs. The CUs are configured to perform no RFfunctionality. The RUs are controlled by the CUs and are implemented byhardware blocks, such as radio transceivers (see, FIGS. 2A and 2B).

The RUs may have transmit antennas that are integral to them or theantennas may be external and connect to the RUs via antenna cables.There may be less software functionality running on the RUs as comparedto the CUs 22, 24. In some implementations, the RUs are configured toperform no baseband modem functionality. In other implementations, theRUs may perform some baseband modem functionality. For example, in theLTE standard, the RUs may implement the Fast Fourier Transform (FFT) andthe Inverse FFT (IFFT) functions. In some implementations, RUs mayperform additional downlink baseband modem functions. The basebandmodems in the CUs and the RUs are connected through a standardoff-the-shelf switched Ethernet network 28 with one or more Ethernetswitches 34, 36, 38. In some implementations, all CUs and RUs at thesite 10 are connected to each other through the Ethernet network 28.

One or more RUs together with a baseband modem in a given CU form aphysical cell. In the example shown in FIG. 1 , a cell 44 includes RUs26 a-26 d controlled by one or more baseband modems (not shown) in theCU 22, and a cell 46 includes RUs 26 e-26 i controlled by one or morebaseband modems (not shown) in the CU 24. The RUs 26 a-26 i can bedeployed at different locations of the site 10, e.g., different rooms,floors, buildings, etc., to provide a RF coverage across the site asuniformly as possible. Each CU may have one or more baseband modems andcan control one or more cells. Nominally each baseband modem has thedata transmission capacity of a single LTE sector. The number ofbaseband modems available at the site determines the data capacity thatcan be delivered to the site.

The radio network 12 of FIG. 1 can be implemented with various airinterface technologies. Currently, 4G LTE is expected to become thedominant wireless technology around the globe. LTE is a standarddeveloped by 3GPP, a standards organization. The first version of theLTE standard was made available in 3GPP Release 8. Subsequently, the LTEstandard was refined in Releases 9 and 10. Release 11 is currently underdevelopment and several more releases of the standard will be developedin the future. In the remainder of this disclosure, we use 3GPP Releases8-11 for the LTE standard as examples in describing the implementationsof the radio networks. However, the radio networks and other systems andmethods of this disclosure can be utilized with any release of the LTEstandard, including Frequency-Division Duplex (FDD) and Time-DivisionDuplex (TDD) variants, or with a variety of other future or existing airinterface technologies, such as the IEEE 802.11, which is more popularlyknown as Wi-Fi, or IEEE 802.16, which is also known as Wi-Max, or even3G air interfaces such as Universal Mobile Telecommunications System(UMTS).

Most commercial LTE networks are synchronous so that the timing phasesof all transmissions from the eNodeBs are aligned with GPS (globalpositioning system) time or UTC (coordinated universal time). In astandalone LTE eNodeB, the GPS/UTC time is provided by a GPS receiver,which is a physical component on the eNodeB hardware. In someimplementations, the hardware of the CUs 22, 24 include a physical GPSreceiver to provide timing to the radio network 12. In deployments wherethe CUs 22, 24 are far away from any satellite view, e.g., located deepinside a building, the physical GPS receiver (not shown) can be externalto the CU hardware and can deliver the timing information to the CUs 22,24 through, e.g., the IEEE1588 PTP (precision time protocol). In someimplementation, a source of timing for the radio network 12 is a timingserver (not shown) located in the operator's network (e.g., the network14, 16) that provides timing to the CUs 22, 24 using, e.g., the IEEE1588protocol. The RUs 26 a-26 i do not necessarily contain any GPS receiver,and receive timing information either from the CUs or directly from anexternal GPS receiver via IEEE1588 or other high-precision timingprotocols. Synchronization is discussed in detail further below.

Referring to FIG. 2A, a CU 60 includes a baseband modem 62 connected toRUs 66 a-66 e through an Ethernet network 68. RUs 66 a-66 e belong tothe same cell 64. The positions of the RUs are chosen to provide RFcoverage, which depends primarily on the transmitter power of the RUsand the RF propagation environment at the site. The data capacity of asingle baseband modem can be shared by all UEs that are in the coveragearea of the RUs that belong to the corresponding cell. The number of RUsto be assigned to a single cell can be determined based on the number ofUEs in the coverage area of the RUs, the data capacity needs of each UE,as well as the available data capacity of a single baseband modem, whichin turn depends on the various capacity-enhancing features supported bythe baseband modem.

In a radio network, the size and shape of the cells can be varied in asite according to the traffic demand. In high traffic areas cells can bemade smaller than in low traffic areas. When traffic demand distributionacross the site varies according to time-of-day or other factors, thesize and shape of cells can also be varied to adapt to those variations.For example, during the day more capacity can be delivered to the lobbyareas of a hotel than to the room areas, whereas at night more capacitycan be delivered to the room areas than the lobby areas.

The RUs 66 a-66 e can provide uniform signal strength throughout thecell 64 without introducing any cell boundaries. When the capacity of asingle baseband modem 62 is insufficient to serve the area, additionalmodems can be added to the CU or unused modems can be enabled in the CUto split an existing cell into multiple cells. More capacity can bedelivered with multiple cells. For example, as shown in FIG. 2B, a CU 80includes modems 82, 84 controlling respective cells 86, 88 through anEthernet network 96. Each cell 86, 88 includes one or more RUs 90 a, 90b, 92 a, 92 b to provide RF coverage to UEs 94 a-94 d. The cells 86, 88can be used by the subscribers of one mobile operator, or by differentmobile operators. If needed, additional CUs with more baseband modemscan also be added. Additional RUs may be added to expand or improve theRF coverage.

In addition to the modems or modem functionalities, the CU 80 contains acoordination unit 98 that globally coordinates the scheduling oftransmission and reception of the modems 82, 84 to reduce or eliminatepossible interference between the cells 86, 88. For example, thecentralized coordination allows devices 94 c, 94 d that are locatedwithin the overlapping boundary region 100 of the two cells 86, 88 tocommunicate without substantial inter-cell interference. The details ofthe centralized coordination are discussed further below. Theinterference issues that are likely to take place in the boundaryregions of multiple cells within the entire building or site occur lessfrequently because of the relatively few number of cells needed. TheCU(s) can readily perform the centralized coordination for therelatively few number of cells and avoid inter-cell interference. Insome implementations, the coordination unit 98 may be used as anaggregation point for actual downlink data. This may be helpful forcombining downlink traffic associated with different cells whenmulti-user MIMO is used between users served on different cells. Thecoordination unit may also be used as an aggregation point for trafficbetween different modem processors that belong to the same basebandmodem.

Unless specified, the discussions below are mostly directed to one cell,and can be readily extended to multiple cells. Referring to FIG. 2C, aRU 200 for use in the radio network of FIGS. 1 and 2A-2B can have twoantennas 202, 204 for transmitting RF signals. Each antenna 202, 204transmits RF signals on one or more LTE channels (or carriers). The cellto which the RU 200 and its antennas 202, 204 belong carries an ID(Cell-ID). The CU and its RUs and antennas may support multiple LTEchannels, each with a different Cell-ID. In addition, each antenna 202,204 is assigned to a unique Release 8 logical antenna port (ports 0, 1,2 or 3) and possibly a unique Release 9/10 logical antenna port (ports15, 16, . . . , 22). For the purpose of discussion, the antennas 202,204 are also referred to as physical antennas, while the logical antennaports are also referred to as virtual antenna ports. In the exampleshown in FIG. 2C, the antenna 202 is assigned to the Release 8 logicalantenna port 0 and the Release 9/10 logical antenna port 15; and theantenna 204 is assigned to the Release 8 logical antenna port 1 and theRelease 9 or Release 10 logical antenna port 16. The logical antennaports, together with the Cell-ID and other parameters configured in theCU, determine the CS-RS (cell-specific reference signal) 206 theantennas transmit under Release 8, or the CSI-RS (Channel StateInformation-reference signal) 208 the antennas transmit under Release 9or Release 10.

The RF signals transmitted by the antennas 202, 204 carry the LTEsynchronization signals PSS/SSS, which include a marker for the Cell-ID.In use, an idling UE monitors the reference signals associated with aCell-ID, which represents one LTE channel in one cell. A connected UEmay transmit and receive RF signals on multiple LTE channels based onchannel aggregation, a feature of the LTE standard defined in Release 10(details discussed below).

The RU 200 can also have more than two antennas, e.g., four, six, oreight antennas. In some implementations, all RUs in the radio network(e.g., the radio network 12 of FIG. 1 ) have the same number of transmitand receive antennas. In other implementations, the RUs have differentnumbers of transmit or receive antennas.

The radio networks described above can be readily upgraded in the CUs,e.g., to support future LTE or other standards, without makingsubstantial changes, e.g., any changes, to the deployed RUs. In someimplementations, when the RUs support multiple frequency channelssimultaneously, an upgrade for carrier aggregation can be performed byenabling additional channels in the same RU or alternatively bydeploying new RUs that add more channels. In carrier aggregation using asingle RU or multiple RUs, the aggregated channels may be in the same ordifferent frequency bands. Likewise, when the RUs support frequencybands for the TDD (time-division duplex) version of the LTE standard,Time-Division (TD)-LTE capability may be added at a later date byupgrading the CU's and possibly the RU's software/firmware, or by addinga new CU. If Wi-Fi support is required, Wi-Fi capability may be added tothe RUs. WiFi transceivers in the RUs can be managed by the same or adifferent controller and can be managed by the same service managers,both at the site and in the operator's network. Such upgrades can beperformed in a cost effective manner, e.g., by making hardware changes(sometimes at most) in a relatively small number of CUs in a centrallocation (as opposed to replacing a large number of RUs that are spreadacross the site).

Radio Network Deployment

Referring to FIG. 3 , a radio network 120 is deployed at a site 122. Oneor more CUs 124 are installed in a room 126, e.g., a telecom room,locally at the site 122. The RUs 128 a-128 l are distributed around thesite 122. In some implementations, some RUs are wall-mounted withintegrated antennas, some RUs are hidden in one or more closets, andsome RUs are installed above the ceiling tile and attach to a wall-mountantenna via an external antenna cable.

The RUs 128 a-128 l connect to the CUs 124 through a switched Ethernetnetwork 130, which includes twisted pair and/or fiber optic cables, andone or more Ethernet switches. Components of the Ethernet network 130are standard off-the-shelf equipment available on the market. In someimplementations, the Ethernet network 130 is dedicated to the radionetwork alone. In other implementations, the radio network 120 sharesthe Ethernet network 130 with other local area traffic at the site 122.For example, in an enterprise network such other traffic may includelocal traffic generated by various computers in the enterprise that maybe connected to the same Ethernet switches. The radio network trafficcan be segregated from other traffic by forming a separate Virtual LocalArea Network (VLAN) and high-priority QoS (Quality of Service) can beassigned to the VLAN to control latency. In the example shown in FIG. 3, the CUs 124 are connected to a co-located Ethernet switch 132 (in thesame room 126). In some implementations, the connection 134 uses asingle 10 Gb/s Ethernet link running over fiber optic or Category 6twisted pair cable, or multiple 1 Gb/s Ethernet links running overCategory 5/6 twisted pair cables.

Those RUs (not shown in FIG. 3 ) that are near the telecom room 126 maydirectly connect to the Ethernet switch 132 in the telecom room 126. Insome implementations, additional Ethernet switches 136, 138, 140 areplaced between the Ethernet switch 132 and the RUs 128 a-128 l, e.g., inwiring closets near the RUs. Each wiring closet can contain more thanone Ethernet switch (like the switch 136, 138, 140), and many Ethernetswitches can be placed in several wiring closets or other rooms spreadaround the site. In some implementations, a single Category 5/6 twistedpair cable is used between a RU and its nearest Ethernet switch (e.g.,between the RU 128 a and the Ethernet switch 136). The Ethernet switches136, 138, 140 connect to the Ethernet switch 132 in the telecom room 126via one or more 1 Gb/s or 10 Gb/s Ethernet links running over fiberoptic or Category 6 twisted pair cables. In some implementations,multiple RUs are integrated into a single physical device (not shown) tosupport multiple frequencies and possibly multiple mobile operators.

Antenna Mapping in a Cell

Referring to FIG. 4A, a cell 300 (controlled by a single modem or asingle CU) contains sixteen RUs 302 a-302 p. The N (an integer, e.g., 1,2, 4, etc.) physical antennas of each RU may be mapped to the same groupof CS-RS or CSI-RS virtual antenna ports 0 . . . N−1. In the exampleshown in FIG. 4A, N is two, and the mapping is done in the same manneras shown in FIG. 2C. All RUs 302 a-302 p in the cell 300 transmit thesame Cell-ID on the same LTE channel, and all antennas share the sameCell-ID and broadcast the same Cell-ID in the Primary and SecondarySynchronization Signals (PSS/SSS). (When a RU serves multiple channels,different channels may be using different Cell-IDs.) When a UE islocated in the cell 300, the UE receives the reference signals of thesame logical antenna port, e.g., port 0, from different physicalantennas of different RUs. To the UE, the RUs appear as part of a singlecell on a single LTE channel.

Alternatively, multiple RU clusters each containing one or more RUs areformed within a single cell. The antennas in the cluster are assigned todifferent CS-RS or CSI-RS virtual antenna ports, but share the sameCell-ID. For example, as shown in FIG. 4B, a cell 320 contains 16 RUs322 a-322 p each having two antennas and eight clusters 324 a-324 f eachcontaining two RUs. Within each cluster 324 a-324 f, the four physicalantennas of the two neighboring RUs are assigned to four different CS-RSvirtual antenna ports 0, 1, 2 and 3 and four different CSI-RS virtualantenna ports 15 through 18. As a result, a cluster having a total of N(N is four in FIG. 4B) physical antennas appears to the user equipmentas a single cell with N transmit antenna ports.

Compared to the cell configuration shown in FIG. 4A, the number ofantenna ports seen by the user equipment is doubled in FIG. 4B. Theconfiguration of FIG. 4B can improve the performance of the UE,especially when the UE is near the coverage boundaries of two or moreneighboring RUs. Assuming that the UE has two antennas for receivingsignals, under Release 8, the UE can communicate with the radio networkthrough 4×2 single-user MIMO. In systems compatible with Releases 9-11of the LTE standard, up to 4 RUs with 2 transmit antennas each can beused to form an 8-antenna cluster, and then the UE can implement 8×2single-user MIMO. The same UE within a radio network having theconfiguration shown in FIG. 4A can communicate through 2×2 single-userMIMO. Even higher order MIMO communication, e.g., 4×4, 8×8, are possiblefor UEs with 4 or 8 receive antennas.

Increasing the number of physical transmit antennas involved in MIMOcommunications, e.g., using the configuration of FIG. 4B, does notsubstantially increase the processing complexity, except when the numberof layers in spatial multiplexing increases, e.g., from 2 (FIG. 4A) to 4(FIG. 4B). Although clusters of two RUs are shown and discussed, asexplained above, a cluster can include other numbers of RUs, and cell320 can include clusters having different sizes.

In some implementations, a wrap-around structure is used by the CU inassigning the physical antennas to logical (or virtual) antenna ports,such that anywhere within the coverage of the cell 320, a UE can receivefrom as many logical antenna ports as possible. This wrap-aroundstructure can allow the single-user closed-loop MIMO to operate insidethe cell 320 seamlessly over a large coverage area.

Downlink Simulcast and Coordinated Transmission

Referring again to FIGS. 4A and 4B, all antennas are assigned to thesame logical (or virtual) antenna port transmit the same referencesignals (CS-RS or CSI-RS) in a time-synchronized manner. The assignmentcan reduce the effects of shadow fading through macrodiversity. Theassignment can also present a multipath channel to each UE (not shown).Under Release 8, a UE reports a single CSI feedback (including CQI(channel quality Indicator) and PMI/RI (pre-coding matrix indicator/rankindicator)) based on the CS-RS or CSI-RS reference signals it receivesfrom all transmitting antenna ports in the cell. When antennas ofdifferent RUs are transmitting the same reference signal, the UE mayexperience richer scattering and a more MIMO-friendly Rayleigh-likechannel without significant interference from other transmit antennas inthe same cell. Furthermore, the UE only sees one cell, and there is noneed for any handoff when the UE is in the coverage area of multiple RUsthat belong to the same cell.

A single broadcast channel PBCH (physical broadcast channel) is used inthe cell 300 or the cell 320. The cells 300, 320 also implement a singledownlink control region for transmitting signals on PDCCH (physicaldownlink control channel), PHICH (physical hybrid-ARQ (automatic repeatrequest) indicator channel) and PCIFCH (physical control formatindicator channel). Other common logical channels, such as the pagingchannel PCCH, that are transmitted over PDSCH (physical downlink sharedchannel) are also shared.

As discussed previously, all physical antennas that are assigned to thesame logical or virtual antenna ports, such as the Release 8 logicalantenna ports and the Release 10 CSI-RS resources, transmit the samecontrol signals and reference signals. In the example shown in FIG. 4B,all PDCCH/PHICH/PCIFCH transmissions use 4-antenna TX diversity and alltransmissions from those antennas assigned to the same logical antennaport are identical. A UE within the cell 320 perceives transmissionsfrom those antennas assigned to the same antenna port as if thetransmissions are delivered from a single antenna through a multipathchannel.

Furthermore, new capabilities in Release 11 can be implemented toimprove the downlink MIMO operation inside a large cell, like the cells300, 320, that has many RUs. In Release 11, multiple non-zero CSI-RSresources can be used inside a single cell. As an example, referring toFIG. 4C, each RU 402 a-402 p (or clusters of RUs) of a cell 400 isassigned to a different CSI-RS resource with a distinct CSI scramblingID 404 a-404 p. Each RU with the distinct CSI scrambling ID operates asif it were a virtual cell, even though they share the same Cell-ID withother RUs in the same cell. The multiple CSI-RS resources (andscrambling IDs) in the cell 400 are monitored by the UE. In someimplementations, the UE can be configured by the CU (not shown, e.g.,the CU 22, 24 of FIG. 1 ) of the radio network to perform the monitoringof multiple CSI-RS resources.

A UE (not shown) in the cell 400 sends multiple CSI reports to the CU ofthe radio network for multiple RUs whose CSI-RS transmissions the UEmonitors. From each CSI report, the CU obtains a CQI for the respectiveRU and uses the CQI for determining signal strength from that RU. The CUcan use these multiple CQI reports along with multiple PMI/RI reportsreceived from the UE to more accurately determine the precodercoefficients. Accordingly, the multiple CSI reports can reduce the CSIquantization error and improve the overall performance of the radionetwork. For example, when a UE reports CSI independently for twoadjacent RUs, such as RUs 402 a, 402 b, the CU determines the precodercoefficients with greater accuracy than when only a single non-zeroCSI-RS resource is reported. Furthermore, Release 11 supports enhancedCQI reporting based on accurate interference measurements by the UE.Release 11 also includes an E-PDCCH (enhanced physical downlink controlchannel), which can be used to increase the control channel capacity inthe cell 400. All these features of Release 11 enhance the functionalityof the present disclosure.

In some implementations where the radio network supports multiple cells,downlink transmissions in different cells can be coordinated to reduceinterference. Coordination may be achieved using techniques such as Hardand Soft Frequency Reuse (HFR/SFR) or Release 11 Coordinated Multipoint(CoMP), which are described in more detail later.

Uplink Diversity Reception

The uplink transmissions by a UE that is being served by a cell withmultiple remote units will be received by all the RX antennas in theseRUs. When the UE is near the coverage boundaries of two or more RUs, itstransmissions may be received by RX antennas of these RUs. In thissituation, the uplink performance can be improved by performingdiversity combining (i.e., Maximal Ratio Combining (MRC), InterferenceRejection Combining (IRC) or Successive Interference Cancellation (SIC)in the controller) across signals received by multiple RUs. By havingmultiple RUs send the received IQ data to the controller,multi-antenna/multi-RU combining can be achieved.

When there are two or more cells in the radio network, uplinktransmissions of a UE that is being served by a first cell may bereceived by the RX antennas of one or more RUs that belong to othercells. In this situation, uplink performance can also be improved byperforming diversity combining (e.g., MRC, IRC or SIC) across signalsreceived by multiple RUs, including the RUs that belong to differentcells.

Virtual Cell Splitting

The capacity in the radio network can be increased by a cell splittingprocedure. In the procedure, RUs in a single cell are split between twocells, increasing the capacity at the site. The two cells can deliver upto twice the capacity because two UEs can be served in two differentcells on the same time-frequency resource.

Alternatively, the capacity of a single cell can be increased by usingvirtual cell splitting. The cells each containing multiple RUs asdiscussed above can be virtually split, by allowing multiple UEs totransmit simultaneously using the same time-frequency resources, usingeither multi-user MIMO, which is an extension of single-user MIMO tomultiple UEs supported in the LTE standard, or RF isolation. In contrastto real cell splitting, virtual cell splitting does not impact thereference signals or common control channels. Virtual cell splittingincreases cell capacity by allowing multiple UEs to transmit or receivedata using the same time frequency resources.

1. Downlink Virtual Cell Splitting A. Multi-User MIMO

In some implementations, virtual cell splitting is implemented withmulti-user MIMO, which is used to send data to multiple UEs on the samePDSCH time-frequency resource. The multiple UEs can be served on thesame time-frequency resource even when these UEs receive strong RFsignals from the same antennas. Multi-user MIMO technique is an integralpart of the LTE standard.

In multi-user MIMO, a unique set of precoder weights is applied tomodulation symbols destined to each UE to prevent interference betweenco-scheduled UEs. In particular, when each UE has a single antenna,individually generalized beams are formed for each UE. When each UE hasmultiple antennas, the CU and the RUs may provide spatial multiplexing(i.e., sending multiple layers of modulation symbols) to each UE, inaddition to serving the multiple UEs on the same time-frequencyresource.

Multi-user MIMO can be used with the antenna mapping schemes shown inFIGS. 4A and 4B. For example, in the antenna mapping scheme of FIG. 4A,two UEs can be served on the same time-frequency resource by one or moreRUs. The CU for the cell 300 forms two beams in directions of thestrongest RF paths for the two UEs, without causing significantinterference between the two UEs.

In Release 8, multi-user MIMO is supported in downlink transmission mode5. Each UE having a single antenna reports to the CU a 2×1 precodingvector selected from a 4-entry precoding codebook and an associated CQI,which is based on single-user beamforming using the selected precodingvector. When the precoding vectors selected by two UEs are orthogonal toeach other, the CU may schedule the two UEs on the same time-frequencyresource using half of the available transmit energy for each UE.

For two UEs that have no inter-user interference cancellationcapabilities, the multi-user MIMO with the antenna mapping scheme ofFIG. 4A does not introduce substantial interference when each UEreceives downlink signals from both antennas of a RU at about the samestrength, and when the selected precoding vectors of the two UEs areorthogonal to each other.

Multi-user MIMO can also be implemented with advanced UEs that arecapable of using knowledge about the modulation structure of theinterfering signals from co-scheduled UEs to reduce the interference. Insome implementations, a UE with two or more antennas can remove part ofthe interference using spatial filtering.

In transmission Mode 8 or 9 of Release 9 or 10, multi-user MIMO can beimplemented using DM-RS (demodulation reference signal), which allowsthe CU to use any precoder without being limited to those precoders thatare defined in the standard in so-called codebooks. The UE reports tothe CU the CSI implicitly by selecting a precoder from a predeterminedcodebook. In some implementations, the UE determines the CSI using theCSI-RS reference signal, which can support up to 8 antenna ports. InRelease 10, the same CSI-RS signal is transmitted from all physicalantennas of the RUs that are assigned to the same CSI-RS logical antennaport and the UE reports only one CSI (i.e., CQI/PMPRI) for each(physical) cell. In Transmission Mode 9, the CU can schedule up to 4 UEson the same time-frequency resource with up to 2 layers per UE and up to4 layers per RB (Resource Block). The CU transmits DM-RS on 12 REs(Resource Elements) per RB and the 12 REs are used for all UEs that areco-scheduled on the same resource. The transmission based on DM-RS canprovide flexibility and simplification in scheduling.

In some implementations, when the CU knows the channel coefficients, itchooses the precoding vectors for the UEs to provide each UE with themaximum SINR (Signal-to-Interference and Noise Ratio) without the UEexperiencing substantial interference. As discussed previously,interference suppression capabilities provided by the UEs can furtherfacilitate reliable multi-user MIMO.

Release 11 supports using multiple CSI-RS signals inside a physical celland allows a UE to send more than one CQI/PMI/RI report per physicalcell. This can improve the performance of the multi-user MIMO. Forexample, in Release 11, each RU (or each group of RUs) may be assignedto a CSI-RS reference signal sequence that is different from thoseassigned to the other RUs in the same cell. Each UE is requested toreport the CSI individually for multiple RUs in the cell. The CQI/PMI/RIinformation obtained from the multiple reports can be more accurate thaninformation obtained from a single report. Based on the accurateinformation, the CU can determine with greater precision the precodingvectors in multi-user MIMO and reduce inter-user interference. In someimplementations, the CU configures each UE with a selected set, e.g.,but not necessarily the entire set, of CSI-RS resources available in thecell so that the UE does not have to send CSI reports for all CSI-RSresources in the cell.

B. RF Isolation

Virtual cell splitting in a cell can also be achieved based on RFisolation among the UEs in the cell. In some implementations, multipleUEs are served simultaneously on the same time-frequency resource viaRUs or antennas whose coverage areas do not substantially overlap. For afirst UE, instead of simulcasting the same PDSCH signal on all physicalantennas that are assigned to the same virtual antenna port, only a fewRUs and physical antennas that provide the strongest signals to thegiven UE are allowed to transmit the signals to the first UE.Transmissions from other RUs and physical antennas to the first UE arepurged. One or more of the RUs that are not transmitting to the first UEcan instead transmit to a second UE on the same time-frequency resource.When the transmissions from the physical antennas of the RUs serving thefirst UE are received at a very low level by the second UE, and likewisewhen the transmissions from the physical antennas of the RUs serving thesecond UE are received at a very low level by the first UE, nosignificant interference occurs, even when the UEs do not have anyinterference suppression capabilities. DM-RS reference signals aretransmitted similarly to the PDSCH signals. For example, the DM-RSreference signals for the first UE are transmitted only from theantennas of the RUs that are serving the first UE. In Release 10,multi-user MIMO can be used to send up to 4 layers to two or more UEs.Generally, such limitations do not apply in the RF isolation method, butin some implementations, additional steps may need to be implemented toreduce or avoid interference between UEs.

In the example shown in FIG. 5A, two UEs 502, 506 at different locationsin a single cell 500 are co-scheduled on the same time-frequencyresource based on RF isolation with up to 2 layers per UE. The cell 500includes 12 RUs 506 a-5061, each having two physical antennas andtransmitting CSI-RS on virtual antenna ports 15 and 16. To serve the twoUEs that are spatially far apart in a given subframe, the single cell500 is virtually split to form three virtual cells 508 a, 508 b, 508 c.The RUs 506 a, 506 b, 506 g, 506 h in the virtual cell 508 a serve theuser equipment 502. The RUs 506 e, 506 f, 506 k, 5061 in the virtualcell 508 c serve the user equipment 506. The RUs 506 c, 506 d, 506 i,506 j in the virtual cell 508 b do not serve any UE in order to avoidcausing interference to the UEs 502 and 506. The total number of layersco-scheduled in the single cell 500 is 4. The virtual cells describedabove are not static like physical cells. The virtual cells can varydynamically from one subframe to the next and across resource blocks. Insome implementations, the dynamic variation applies only to the shareddata channel PDSCH. For example, there may be no virtual cell splittingin one subframe, while in another subframe, two different virtual cellsplitting may be applied in two different groups of resource blocks. Insome implementations, a virtual cell may have a single RU withoutsimulcasting, which can eliminate the intentional multipath caused bysimulcasting. The virtual cells represent the ability of the system toserve multiple UEs in the same cell on the same time-frequency resource.

The RUs within the same virtual cell transmit the same DM-RS referencesignal selected from four available ports/scrambling index {7.0, 7.1,8.0, 8.1}. The virtual cells that are located adjacent to each other (orclose to each other without directly bordering each other), such as thevirtual cells 508 a, 508 b and the virtual cells 508 b, 508 c, usedifferent DM-RS port numbers. Those virtual cells that are relativelyfar apart, e.g., the virtual cells 508 a, 508 c, can reuse the sameDM-RS reference signal based on the RF isolation. In suchimplementations, signal transmissions between the UEs and the radionetwork are performed without significant interference between thevirtual cells.

In some implementations, the CU chooses a MCS (Modulation and CodingScheme) for each co-scheduled UE based on the CQI values, determined bythe UE from the CS-RS or CSI-RS signals, reported by the UE. The CS-RSor CSI-RS signals are transmitted continuously by all physical antennasin the physical cell, including some antennas that may at times nottransmit in the shared data channel PDSCH. The CS-RS or CSI-RS signalstransmitted from the physical antennas that are near the UE, whenreceived at sufficiently high strength, are seen by the UE as multipletransmission paths, or RF multipath. In some implementations, the UE canpredict a higher (or lower) CQI based on the multipath than the actualCQI the UE will experience when receiving on PDSCH with less multipath.In such implementations, the HARQ (hybrid automatic repeat request)capability in the LTE standard can provide dynamic adaptability toreduce the effect caused by the mismatch between the predicted CQI andthe actual CQI. In some implementations, when the actual channelconditions are worse than the conditions predicted by the CQI, the CUretransmits the data or signals with incremental redundancy to achievethe maximum data rate that the channel can support.

The virtual cell splitting techniques described above using multi-userMIMO or RF isolation can be utilized in systems that are compatible withall Releases of the LTE standard. Release 8 UEs use CS-RS, instead ofDM-RS, for demodulation, which in some situations, may cause mismatchduring demodulation. Still in many cases, virtual cell splitting in themanner described above is highly desirable when there is a strong RFisolation between the transmitting and the non-transmitting antennassuch that the UEs can achieve total throughput higher than when eitherUE is served on a dedicated time-frequency resource.

In Releases 9 and 10, in some implementations, the single CQI/PMI/RIfeedback sent by the UEs may not be sufficient for the CU to determinereliably which RUs and physical antennas are most likely to provide thestrongest signal to each UE (in the downlink direction). In suchimplementations, the CU can also use information about the strength ofuplink signals, such as the Sounding Reference Signal (SRS) or PUCCHcontrol signals or PUSCH uplink data, received by the RUs from the UEsto determine the antennas that are likely to provide the strongestsignal to each UE on the downlink. After the CU determines the RUs orphysical antennas for transmission to a given UE, the CU chooses theprecoding vector weights as described above so that signals to a UE aretransmitted from antennas that the UE hears strongly.

The virtual cell splitting using RF isolation can be implemented withhigher accuracy in Release 11, where the UEs are capable of sendingmultiple CQI reports for different RUs. The CU uses these CQI reports todetermine which RUs or physical antennas transmit signals that arelikely to be received by co-scheduled UEs at a high strength.

2. Uplink Virtual Cell Splitting

Referring again to FIG. 5A, it is possible to implement virtual cellsplitting also on the uplink. The CU may schedule multiple UEs on thesame time-frequency resource and reduce or remove any interferencebetween co-scheduled UEs in the CU using Interference RejectionCombining (IRC) or Successive Interference Cancellation (SIC). Thesetechniques can rely upon spatial filtering as in multi-user MIMO or asin RF isolation. On the uplink, the UEs 502, 504, 506 share certainuplink resources that are available in the cell 500. The uplinkresources can include the cyclic shift for DM-RS reference signals andthe Orthogonal Cover Code (OCC) that are assigned to UEs for PUSCH(Physical Uplink Shared CHannel) transmissions and the resource indicesassigned to UEs for PUCCH (Physical Uplink Control CHannel)transmissions. The CU can create virtual cells on the uplink by reusingthe same resources among UEs in the same physical cell. The number ofUEs that can simultaneously transmit on the same time-frequency resourceis limited at least partially by the availability of the uplinkresources in the single cell. Reusing the same resources among UEs canincrease the total capacity available on the uplink.

A. PUSCH Transmissions

The DM-RS reference signals used by a UE depend on the number ofResource Blocks (RBs) assigned to that UE. For PUSCH transmissions, thenumber of RBs can be as high as 108. A DM-RS reference signal having alength of 12×N is derived from a base sequence of the same length, whereN is the number of RBs assigned to the UE. Up to 12 DM-RS referencesequences (or interchangeably, signals) can be derived from each basesequence using a cyclic shift in the time domain. Thesecyclically-shifted reference sequences are orthogonal to each other.When the channel for transmitting the reference sequences issufficiently flat across one RB, two UEs can transmit their DM-RSreference signals with different cyclic shifts on the same RB. The CUcan then estimate respective uplink channels for the transmissions fromthe two UEs without experiencing any substantial interference betweenthem. When the channel is not sufficiently flat, fewer than 12orthogonal DM-RS reference sequences can be generated by cyclicallyshifting a base sequence.

In some implementations, the orthogonal DM-RS reference sequences areused for single-user spatial multiplexing (up to 4 layers) andmulti-user MIMO. In Release 10, an orthogonal cover code can be appliedto the two DM-RS sequences such that two layers can be transmitted usingthe same cyclic shift, while keeping the DM-RS reference signalsorthogonal.

In some implementations, the UEs that are served by the same physicalcell (e.g., the cell 500 of FIG. 5A) use the same base sequence forPUSCH transmissions. When multiple UEs transmit on the sametime-frequency resource, the CU coordinates the assignment of cyclicshifts and the orthogonal covers in uplink scheduling to keep the DM-RSreference signals transmitted on the same time-frequency resourceorthogonal. In such implementations, a sufficient number of cyclicshifts remain available for the assignment and for use in spatialmultiplexing or multi-user MIMO in each cell. For example, when 6 cyclicshifts of the base sequence are available and the 6 cyclic shifts arecoupled with a pairwise orthogonal cover code, the CU can serve as manyas 12 layers on the same uplink time-frequency resource with orthogonalDM-RS reference signals.

In some implementations, a physical cell described previously (e.g., thesingle cell 500 of FIG. 5A) can be arbitrarily large. In a large cell,when there is extensive use of simultaneous uplink transmissions on thesame time-frequency resource, the CU may be short of available cyclicshifts and orthogonal covers to maintain the orthogonality among theDM-RS reference signals. Similar to the RF isolation on the downlink,the uplink can reuse the one or more DM-RS reference signals on the sametime-frequency resource when the uplink transmissions by theco-scheduled respective UEs do not substantially interfere with eachother. In some implementations, when there is no substantial overlapbetween signals received from the co-scheduled UEs by certain groups ofRUs or receive antennas, the same DM-RS reference signal can be used forthose UEs. The CU can determine which groups of receive antennas or RUsare receiving significant signals from a UE based on PUCCH, SRS(Sounding Reference Signals) and prior PUSCH transmissions, and canassign cyclic shifts and OCCs accordingly.

In some implementations, when there are multiple cells served by one ormore controllers, it is also possible to assign the same base sequenceto all cells. This allows the controller to assign all UEs to cyclicshifts of the same base sequence and to ensure orthogonality betweenUEs, including those UEs that are served by different cells. Based onthe RF isolation, the controller can also reuse the same cyclic shiftsin different parts of the site and increase the number of UEs that canbe supported.

In a radio network compatible with the Release 11 standards, differentRUs in a cell (such as the cell 500 of FIG. 5A) may be assigned todifferent DM-RS base sequences. In some implementations, orthogonalitybetween different cyclic shifts of different base sequences is notguaranteed, but the number of available DM-RS sequences is increased.Accordingly, the size of the cell can be increased and more UEs can beserved on the same time-frequency resource.

B. PUCCH & PRACH Transmissions

For PUCCH transmissions, for example for transmitting HARQ ACK/NAKs orChannel State Information (CSI), different UE transmissions in differentcells use different base sequences to avoid collisions among UEtransmissions in the different physical cells. This can be achieved byensuring that the Cell-IDs used by neighboring cells do not overlapmodulo 30. Group hopping, a feature of the LTE standard, can also beused to randomize the interference between the PUCCH transmissions fromdifferent UEs in different physical cells.

Orthogonal cyclic shifts of the base sequences (and possibly OCCs) areused in PUCCH transmissions to allow multiple UEs to transmit on thesame time-frequency resources. In some implementations, it is possibleto reuse the cyclic shifts (and OCCs when used) in different parts ofthe cell to increase the number of UEs that transmit at the same time.RF isolation can be used by the controller to determine which UEs mayreuse the one or more base sequence cyclic shifts and orthogonal coversfor the same time-frequency resource based on transmissions receivedfrom the UEs, for example, in PRACH (Physical Random Access CHannel) orprevious PUCCH or PUSCH transmissions.

The interference between a cell (e.g., any single cell describedpreviously) and any nearby macro cells (e.g., a mobile network providingcoverage outside the site 10 in FIG. 1 ) is randomized and kept small.In some implementations, the CU chooses base sequences for use in PUSCHor PUCCH transmissions that are different from the base sequences usedin nearby macro cells. Furthermore, the CU can also implement grouphopping.

In some implementations, it is also possible for two or more UEs thattransmit on the Random Access Channel (RACH) using the same preamble tobe detected by the radio network of the present disclosure. Each cellwill have 64 preambles available in every PRACH opportunity. Byindividually processing the received signals from each RU or group ofRUs, the controller may reliably detect multiple PRACH transmissionsthat use the same preamble and that are free of significant interferenceamong them. For example, referring to FIG. 5B, the controller 550individually processes the signals from each RU or group of RUs (e.g.,virtual cells 508 a, 580 b, 508 c) to detect multiple PRACHtransmissions 552, 554, 556 that use the same preamble.

Dynamic Coverage and Capacity Adjustment

Referring again to FIGS. 2A and 2B, the RF coverage and capacityprovided in the radio network are decoupled. The RUs 66 a-66 e, 90 a, 90b, 92 a, 92 b provide the coverage and the baseband modems 62, 82, 84,or the CUs 60, 80 provide the capacity. In some implementations, someRUs in a radio network are deployed more densely and with moretransmitter power than other RUs in order to overcome possibleinterference from nearby eNodeBs, for example, macro cells. In someradio networks of this disclosure, RUs are deployed very closely to eachother, because they can belong to the same cell and therefore do notcause any inter-cell interference. Such very dense deployments aresometimes not possible with traditional base stations. The number ofbaseband modems (and cells) needed for a site depends on the number ofusers, the amount of data usage per user, and the distribution of usersacross the site as a function of time, etc. In general, a minimum numberof baseband modems (and cells) is used to keep the cost low and to avoidunnecessary cell boundaries. When the demand for coverage and/orcapacity changes, the radio network of this disclosure can dynamicallyadjust its coverage and capacity.

1. Dynamic Capacity Reallocation

In some implementations, when multiple RUs share the same cell/basebandmodem, the capacity of the baseband modem is shared by all the UEs thatfall within the coverage area of all the RUs that are assigned to thebaseband modem. In an area of relatively high data usage, the RUs thatform the cell may cover a smaller area than RUs in another cell thatcovers an area of relatively low data usage. For example, at a siteusing 4 modems (and 4 cells) and 24 RUs, the 4 cells can have 2, 4, 8and 10 RUs, respectively, providing different cell sizes that match thecoverage and capacity demand. The assignment of RUs to the cells can bedynamically changed based on changes in capacity demand. The changes canbe made manually, e.g., by having a local person modify the RU tocontroller mapping, semi-automatically, e.g., based on Time-of-Day(ToD), or automatically, e.g., by the controller based on detecting achange in traffic distribution. The changes can reallocate the capacityat the site, without any substantial changes to the deployed equipment.

As an example, referring to FIGS. 6A and 6B, a radio network 602including three modems 604 a, 604 b, 604 c controlling three respectivecells 608 a, 608 b, 608 c through an off-the-shelf Ethernet network 606is deployed at a site 600. The site 600 can be a commercial buildingthat includes shopping areas and office space, which have differentcapacity demands (as schematically shown by different numbers of usersin the figures) at different ToD. The cells may each include differentnumbers of RUs (not shown) to cover different-sized areas, whileproviding substantially the same traffic capacity. The shapes of thecovered areas by the different cells can also be different.

Referring particularly to FIG. 6A, at a given time (time 1, e.g., workhours on a weekday), most users of the site 600 are concentrated insmall areas 610, 612 (e.g., office spaces), while the user density isrelatively low in the larger area 614 (e.g., the shopping areas). Tomeet the different capacity demands in the different areas of the site600, the cells 608 a, 608 b having a relatively small number of RUs areformed to cover the areas 610, 612, and the cell 608 c having arelatively large number of RUs is formed to cover the area 614. Eachcell 608 a, 608 b, 608 c has substantially the same capacity.

The capacity demands at the site 600 may dynamically change. Referringto FIG. 6B, at another given time (time 2, e.g., lunch hours on aweekday), there is a high density of users in areas 618, 620 (e.g.,restaurant areas in the shopping area 614 of FIG. 6A) and there arerelatively few users are in the area 616 (e.g., office areas 610, 612and store areas in the shopping area 614 of FIG. 6A). In response, oneor more RUs at the site 600 are reassigned to different modems,manually, semi-automatically, or automatically, to form new cells 622 a,622 b, 622 c that cover the respective areas 616, 620, 618. The cell 622a contains a relatively large number of RUs. The cells 622 b, 622 ccontain a relatively small number of RUs. Each cell 622 a, 622 b, 622 chas substantially the same capacity. Dynamic capacity reallocation isimplemented over the Ethernet network.

2. Total Capacity Increase

In some implementations, instead of or in addition to redistribution ofcapacity demands on a site (e.g., the site 600 of FIGS. 6A and 6B), thesite also experiences an increase in the demand for total capacity. Forexample, the number of mobile subscribers increases, and/or the amountof data demand per subscriber increases. In these implementations,additional modem(s) (and accordingly additional cell(s)) can beintroduced. For example, an existing unused modem in a CU of the radionetwork can be enabled and some of the RUs already deployed at the sitecan be reassigned to the new modem. This is a form of real cellsplitting, which can be implemented in a convenient manner, e.g., as asoftware upgrade, and typically does not require any hardware changes tothe installed RUs. Alternatively or in addition, one or more new modemscan be added in a CU and/or one or more new CUs can be added to theradio network at the site. In some implementations, the total capacityof the site may be increased without affecting the previously deployedmodems, cells, and RUs. The addition of more modems or CU hardware issignificantly less expensive, both in terms of equipment andinstallation cost, as compared to adding many new access points acrossthe site. The physical cell splitting method described above isimplemented using the Ethernet network.

Downlink Inter-Cell Interference Control

In some implementations, inter-cell interference on PDSCH is reducedusing hard frequency reuse (HFR). HFR can be implemented as a static orsemi-static scheme, where the available resource blocks are dividedbetween groups of cells according to K-way frequency reuse, where K istypically 3 or 7, so that each cell uses one-third (or one-seventh) ofthe available resource blocks. When only one cell transmits in eachresource block, cells in the same frequency reuse group will not see anyPDSCH interference from the others. Implementing HFR may cost(K−1)/K×100% of the available bandwidth.

Alternatively, inter-cell interference on PDSCH can be reduced usingSoft Frequency Reuse (SFR). In SFR, available resources are partitionedbetween neighboring cells in the same frequency reuse group. Differentfrom HFR where each resource block is assigned a binary state (on/off),i.e., full power or no power at all, in SFR, each resource block can beassigned any transmit power level. For example, consider the followingscheme with 3 different power levels (high (H), medium (M), low (L)).Referring to FIG. 17A, in every cell 2400 a, 2400 b, 2400 c, eachresource block 2402 is assigned to one of these power levels (H, L, orM), such that in resource blocks where a cell is assigned a high power,its two neighboring cells are assigned a low power. As a result, eachcell will have two times as many low-power resource blocks as high-powerones. Each eNodeB will assign the UEs that it is serving to one of thepower levels, typically during connection set up, based on the averageSNR the UE is experiencing and possibly other factors such as the amountof data the UE has for transmission. The UEs that are in goodconditions, e.g., located near the center of a given cell, or that havelittle data to send are assigned a low PDSCH power level, whereas UEs inpoor conditions, e.g., located near the cell edge or having a lot ofdata for transmission are assigned a high PDSCH power. Accordingly, whenthe controller is serving a cell edge user, the UE will experience botha higher received signal power and a lower interference power level,boosting its average received SNR. When the UEs move and their channelconditions change, the controller can change the transmit power levelfor the UE by sending a higher layer reconfiguration message. Whenscheduling UEs for transmission on resource blocks, the controller mayeffectively need to run parallel schedulers, one per power level. Insome implementations, the strict partitioning of the resources may leadto scheduling efficiency loss, for example, due to loss of somemulti-user diversity. Such inefficiencies can become visible when thePDSCH power distribution of active UEs is mismatched relative to thepower distribution of the resource blocks. Fixed power allocation canalso be inefficient because it sometimes unnecessarily forces a lowpower transmission for a UE, even though a transmission at a higherpower level may not cause any interference to a cell edge UE served by aneighboring cell in the same frequency reuse group when the UE is on theopposite side of the neighboring cell.

Coordinated Scheduling

The efficiencies of SFR can be improved by implementing theresource/power partitioning dynamically as part of a centralizedmulti-cell scheduler in the controller. The controller can dynamicallyallocate resource blocks and transmission power based on Radio ResourceManagement (RRM) reports received from the UEs. The implementation canavoid the need to assign transmit power levels to resource blockssemi-statically as in HFR or SFR.

In LTE, each cell will periodically broadcast its NeighborList in aSystem Information Block (SIB) Type 4 (SIB4). A connected UE willmonitor the cells in the NeighborList and send Measurement Reports tothe serving cell. These reports can be sent periodically or based oncertain triggers. The reporting period and the triggers are configuredby the serving cell using an RRC-Reconfiguration message. Each UE'sMeasurement Report includes two measurements per cell: i) ReferenceSignal Received Power (RSRP) and ii) Reference Signal Received Quality(RSRQ). RSRP is the average received power of a CS-RS RE and isindicative of the received signal strength, and RSRQ is an additionalsignal quality indicator, which also provides a crude measure ofinterference. In some implementations, coordinated scheduling in thecontroller will work as follows:

Each baseband modem will send to the central coordinator the NeighborList RSRP reports received from each of the connected UEs it is serving,as well as the amount of data each UE has waiting for transmission.Baseband modems may send these reports upon certain event triggers, forexample when a UE is newly connected or disconnected, or when there is asignificant change in the UEs RSRP reports. It is also possible for thecentral coordinator to poll the baseband modems to get these RSRPreports.

Central coordinator will use the received information to construct abandwidth and PDSCH power allocation map for each UE and willperiodically send this information to their serving baseband modems. Thebasic logic for creating this bandwidth allocation map is discussedbelow.

Individual cell modems communicate the PDSCH power allocation to theUEs, e.g., shortly after setting up the connection. For every subframe,individual baseband modems schedule UE data for transmission on PDSCH.Baseband modems schedule transmissions in a manner that is consistentwith the power levels and the bandwidth resources allocated to each UEby the central coordinator.

Next, examples using two adjacent cells are provided with FIG. 17B.Suppose each cell 2410 a, 2410 b has one connected UE, and each UE hassimilar amounts of data waiting for transmission. If both UEs are awayfrom cell boundary, the central coordinator would allocate the fulltransmission band to both UEs since neither would experience significantinter-cell interference. If both UEs are near the cell boundary, thenthe cell coordinator would allocate 50% of the transmission bandwidth toeach UE at full power. If one UE is near the cell boundary but the otheris away from the cell boundary, then the cell coordinator could allocatethe full transmission band to both UEs, but assign a lower power levelto the UE away from the cell boundary to reduce interference with the UEnear the cell boundary in the other cell. When the UEs havesignificantly different amounts of data waiting for transmission, thecell coordinator may give more bandwidth to the UE with more data.

In a more complex case where each cell has 10 connected UEs with 50%near the cell boundary and 50% away from the cell boundary and UEs nearcell boundary have similar amounts of data as the UEs away from the cellboundary, central coordinator could allocate resources as follows: UEsthat are away from the cell boundary are allocated the full transmissionbandwidth, but at a reduced power level and UEs near the cell boundaryare allocated 50% of the transmission band in a non-overlapping manner,but at full power. This is illustrated in the diagram below.

If the ratio of the number of UEs at cell edge to the number of UEs atcell center is different from 1:1 or the amount of data the UEs have fortransmission near cell edge is different from the amount of data the UEshave for transmission at the cell center, the central coordinator canadjust the bandwidth and power allocation scheme to match the data needsof the UEs. The adaptability of the allocation can make the systemsignificantly more bandwidth-efficient, while improving the cell-edgeperformance for disadvantaged UEs.

In some cases, there may be interference between the radio network andother networks, such as the macro network, and such interference is alsoconsidered and reduced. Release 8 supports messages in the X2 interfaceto allow eNodeBs to exchange information on power levels that are usedin each of the resource blocks of the eNodeBs. In some implementations,the X2 interface is used between the controller of the disclosure andeNodeBs of the other radio networks (e.g., macrocells). The use canfacilitate exchange of information between the controller and theeNodeBs to support coordinated scheduling. As an example, each eNodeBcan indicate to the controller for each resource block whether the powerlevel in that resource block will remain below a certain threshold,which is also separately signaled. This will allow the controller toschedule those UEs located at cell edges in resource blocks where theneighboring cells are transmitting below a certain power level. Similartechniques can be used to coordinate transmissions by differentcontrollers in the same radio network, in which each controller can beinformed about the SFR (Soft Frequency Reuse) power assignments via amanagement system or using a variant of the X2 interface.

Interference Control Techniques for Release 10 UEs

In some implementations, inter-cell control channel interference forhierarchical networks with closed access or range extension can bereduced by having the cells turn off (blank) power in all resourceblocks in certain subframes. When no PDSCH data is transmitted in asubframe, there is also no control messages sent on the downlink controlchannel, which significantly reduces PDCCH interference. Moreover, byconfiguring these blank frames as so-called MBSFN (Multicast/BroadcastSubframes), one can also eliminate interference from CS-RS REs in thePDSCH region.

In an MBSFN subframe, CS-RS is only transmitted in the control region ofthe subframe. This at least eliminates the CS-RS interference into PDSCH(although not necessarily to PDCCH) transmissions in neighboring cells.MBSFN subframes in LTE were developed in Release 8 to carrybroadcast/multicast signals, but they can also be used to send no dataat all. A cell can be configured to send MBSFN subframes according to acertain pattern, and the pattern can be communicated to UEs via theSystem Information Block (SIB). Only 6 out of 10 subframes (#1, 2, 3 and6, 7, 8) in a radio frame can be used for MBSFN. MBSFN frames have acontrol region of up to 1 OFDM symbol for 1 or 2 TX antennas and 2 OFDMsymbols for 4 TX antennas.

Using blank MBSFN subframes alone may not eliminate inter-cellinterference between PBCH, system information (SIB) and PSS/SSStransmissions. In some implementations, the inter-cell interference isbetween a small cell and a single macro cell, and the interference canbe reduced or eliminated by offsetting the subframe numbering in thesmall cell relative to the macro cell. For example, if the relativesubframe number of the small cell network has an offset of 2 relative tothe macrocell network (i.e., subframe #0 in small cell network coincideswith subframe #2 in the macrocell network), and macrocell subframes 2and 7 are ABS/MBFSN subframes, small cell UEs can receive PSS/SSS andPBCH without any interference from the macrocell.

In some implementations, the macro cell coordinates its transmissionsonly with the controller and it is not necessary for the macro celleNodeB to coordinate its transmissions with multiple base stations.

Coordinated MultiPoint (CoMP)

CoMP refers to a broad set of techniques that involve coordinationbetween neighboring cells to reduce the effects of inter-cellinterference. Full-blown coordination is referred to as JointTransmission (JT). In JT, two or more baseband modems cooperate to servetheir UEs via all RUs that they jointly control. All available antennascan be used to serve one UE with Single-User MIMO or multiple UEssimultaneously using Multi-User MIMO. In some cases where JT isimplemented, UEs send CSI feedback not only for the antenna ports oftheir serving cell, but also for antenna ports of neighboring cells.

In JT, similar to the single-cell multi-user MIMO, transport blocks fordifferent UEs are processed in parallel and then combined before theIFFT. However, different baseband modems handle the processing oftransport blocks of UEs in different cells. In some implementations, thecontroller may include a coordination unit for coordinating schedulingin different baseband modems. The coordination unit may also serve as anaggregation point for combining processed transport blocks originatingin different baseband modems. As an example, a radio network 2700 shownin FIG. 20A includes three cells formed by baseband modem 2706 andremote unit(s) 2716, baseband modem 2708 and remote unit(s) 2718, andbaseband modem 2710 and remote unit(s) 2720. The controller 2704controlling the three cells includes a coordination unit 2702, thatserves as an aggregation point for combining (represented by the symbol“(”) transport blocks originating from different modems 2704, 2708,2710.

Alternatively, as shown in FIG. 20B, in a radio network 2730, basebandmodems 2732, 2734, 2736 controlling cells that including remote unit(s)2742, remote unit(s) 2744, remote unit(s) 2746, respectively, maydirectly exchange data among themselves so that each baseband modem cancombine all signals destined to the UEs (not shown) they serve.

In some implementations, referring to FIG. 20C, in a radio network 2760,each baseband modem 2762, 2764, 2766 sends processed transport blocks tothe RUs 2772, 2774, 2776 and the RUs perform the combining beforeapplying the IFFT.

A somewhat reduced CoMP capability is called Dynamic Point Selection(DPS). In DPS, the serving cell sends PDSCH transmission on atime-frequency resource via only one cell TX antennas based on feedbackcell selection received from the UE. The selected cell can be varieddynamically from one subframe to the next, and even between resourceblocks within the same subframe. The selected cell may be different fromthe serving cell of the UE.

Another form of CoMP is Coordinated Beamforming (CB). In CB, when aserving cell is transmitting to a UE from its RUs, it also accounts forinterference it will be creating for another UE in a neighboring cell.By choosing the precoding vector(s) to null the interference to theneighbor cell UE, the controller allows the baseband modem of aneighboring cell to serve the other UE at a higher data rate.

Release 11 has new capabilities to support coordinated transmission. Forexample, Release 11 allows UEs to report CSI for multiple CSI-RS, whichmay belong to different cells.

Communications Between the Controllers and the Remote Units

As explained previously, the CUs and the RUs of a radio network areconnected through a switched Ethernet network (see, e.g., FIG. 3 ).Nominally, the interface between the CUs and the RUs will carrytime-domain IQ symbols (sometimes also referred to as signals) inEthernet frames. However, the bit rate of the time-domain IQ symbols maybe too high for an Ethernet network. In some implementations, instead ofsending the time-domain IQ symbols a compressed representation of thetime-domain IQ symbols is sent to reduce the bit rate and to provide adata rate between the CUs and the RUs that is compatible with thelow-cost switched Ethernet network. In some implementations, on thedownlink, the CUs of the radio network send the IQ symbols when they arein the frequency-domain and prior to performing the IFFT (inverse fastFourier transform) on the frequency-domain IQ symbols. A CU sends thefrequency-domain IQ data representing each OFDM symbol to an RU, forexample, by quantizing the real and imaginary components of thefrequency-domain symbols. The quantizer output bits are then packetizedin Ethernet frames and transmitted to the RUs over the Ethernet network.The RU reconstructs the quantized frequency-domain IQ symbols beforeapplying the IFFT, inserting a cyclic prefix and performing thefiltering, modulation and RF processing.

For the purpose of discussion, a radio network for a 10 MHz FDD LTEsystem is used as an example. For each TX antenna port, each OFDM symbolhas 600 subcarriers and there are 14 OFDM symbols in every 1 mssubframe. Each subframe has 8,400 Resource Elements (REs) in total. EachRE corresponds to one subcarrier in one OFDM symbol. On the downlink,the first 1-3 OFDM symbols in a subframe are primarily used for controlsignaling (e.g., PDCCH, PHICH, and PCFICH) and the remaining OFDMsymbols carry primarily user data on the shared data channel (PDSCH).Reference signals and other common channels are spread across thetime-frequency axis.

Compressing the IQ symbols in the frequency domain can reduce the bitrate of the traffic sent over the Ethernet network. The compressedfrequency-domain IQ symbols are transmitted over the Ethernet networkwithout guard band zeros or any cyclic prefix. When the CU uses a 12-bitquantizer to compress the frequency-domain IQ symbols, the nominal bitrate of the frequency-domain IQ stream is about 403 Mb/s for 2 TXantennas and 806 Mb/s for 4 TX antennas. This represents a 45% reductionin bit rate compared to quantizing the time-domain IQ stream using thesame quantizer (735 Mb/s for 2 TX antennas and 1471 Mb/s for 4 TXantennas). The rate between the CU and the RUs is reduced and the CU andthe RUs are allowed to communicate through Ethernet links operating at aspeed in the order of Gb/s with less latency.

On the uplink, in addition to RF processing and demodulation, the RUsremove the cyclic prefix from the time-domain IQ samples for eachreceived OFDM symbol and apply the FFT to produce the frequency-domainIQ symbols. The information carried by the symbols is then quantized,packetized in Ethernet frames, and transmitted to the CU over theEthernet network. When the 12-bit quantizer is used, the resulting bitrate of the frequency-domain IQ symbols on the uplink is substantiallythe same as that discussed for the downlink.

Described below are several more techniques that can furthersignificantly reduce the data rate between the CU and the RUs.

1. Downlink Compression within a Cell

A. General Description of the Downlink Compression

Typically all antennas of the RUs that belong to the same antenna portin the same cell (unless explicitly specified as a virtual cell, thecells are physical) transmit the same LTE signal. Accordingly, on thedownlink, for each antenna port the CU sends the same frequency-domainIQ symbol to each RU in the cell. The frequency-domain IQ symbols thatthe CU needs to send to the RUs include the CS-RS and CSI-RS referencesignals, the control channels PUCCH, PCIFCH and PHICH, the shared datachannel PDSCH, and the common channels PBCH and PSS/SSS.

In some implementations, the CU performs a simple form of compression bybroadcasting the frequency-domain IQ symbols to all RUs in the cellusing broadcast Ethernet frames. To implement the broadcast, all RUs inthe same cell are configured to belong to the same VLAN (virtual localarea network). The CU sends to its nearest Ethernet switch an Ethernetbroadcast frame that carries an ID of the VLAN. The Ethernet switch inturn sends the Ethernet broadcast frame to all the RUs on the VLAN thatare directly attached to the Ethernet switch and to other Ethernetswitches that provide paths to other RUs on the same VLAN. In suchimplementations, traffic load on the Ethernet switches on the downlinkdoes not grow with the number of RUs that belong to the same cell.

The broadcast on the Ethernet network and the implementation of theVLANs can simplify processing complexity and reduce the data ratebetween the CU and the Ethernet network. The reduction in the data rateis desirable to reduce the Ethernet frame size and latencies at theswitches.

For the purpose of discussion, the previously introduced example of aradio network implementing the 10 MHz FDD LTE with 2 TX antennas is alsoused as an example in the discussion below. The 8,400 frequency-domainIQ symbols in each 1 ms subframe are organized in the form of a resourcegrid that has 600 OFDM subcarriers in 14 OFDM symbols. The 14 OFDMsymbols are split into two time slots each having a length of 0.5 ms.Each time slot is further split into 50 PRBs (physical resource blocks),each containing 84 frequency-domain IQ symbols arranged in the form of a7×12 grid. In some implementations, each PRB carries at most one PDSCHmixed with reference signals, such as the CS-RS and the CSI-RS. The PRBscan also carry one or more LTE downlink control channels PDCCH, PHICH orPCFICH, or the common channels PSS/SSS and PBCH, mixed with the CS-RSand the CSI-RS.

The downlink frequency-domain IQ symbols are discrete-amplitude symbolschosen from a signal constellation. The PSS/SSS is carried onfrequency-domain IQ symbols that lie on a circle. The PDCCH, PCFICH,PBCH, CS-RS, CSI-RS and DM-RS are carried on frequency-domain IQ symbolschosen from a QPSK/BPSK signal constellation. Without precoding, thefrequency-domain IQ symbols that carry the PDSCH are chosen from a QPSK(quadrature phase-shift-keying), 16-QAM (quadrature amplitudemodulation), or 64-QAM signal constellation. The PDSCH modulation orderis chosen based on the signal quality reported by a UE. In the presenceof precoding, the frequency-domain IQ symbols that carry PDSCH are basedon the product of a precoding matrix with an input vector, whosecomponents are symbols chosen from a QPSK, 16-QAM, or 64-QAMconstellation.

The CU can choose downlink frequency-domain IQ symbols directly from adiscrete-amplitude QAM constellation or by applying a matrix operationto symbols chosen from a discrete-amplitude QAM constellation. Theaverage energy of the frequency-domain IQ symbols can vary betweendifferent downlink channels, but is fixed for a given channel within aResource Element Group, or REG (for control channels) or a PRB (forPDSCH). A REG is a group of 4 consecutive REs in an OFDM symbol. In someimplementations, the PDSCH on the 4^(th) OFDM symbol of the slot canhave a different average energy level from those fixed average energylevels.

B. Methods of Compressing the Frequency-Domain IQ Symbols

The symbols transmitted between the CU and the RUs can be compressed invarious ways. In the discussion below, the first three methods, MethodsI, II, and III, are based on quantization, and the fourth method, MethodIV, is based on modulation-level compression.

I. Fixed Quantization

In this method, the frequency-domain IQ symbols are quantized using afixed uniform scalar quantizer having a fixed rate R and a fixed stepsize Δ. The step size is selected by the CU based on the expectedprobability distribution of the frequency-domain IQ symbols. Inimplementations, the CU quantizes the real and imaginary components ofthe frequency-domain IQ symbols serially and transmits the binary datarepresenting the quantized IQ symbols for each TX antenna to the RUs.The values of R and Δ are sent to the RUs when the RUs initially connectto the CU. The RUs use the information about the rate R and the stepsize Δ to reconstruct the frequency-domain IQ symbols based on the datareceived from the Ethernet network. In some implementations, when thereis a major change in configuration of the radio network that changes Rand/or Δ, the CU sends the modified R and/or Δ to the RUs. In theexample with the 10 MHz FDD LTE having 2 TX antennas per RU and a fixed12-bit quantizer, the quantized frequency-domain IQ stream has a datarate of 403 Mb/s between the CU and the RUs.

II. Adaptive Step-Size Quantization

Instead of applying a fixed quantizer step size Δ, in this method, thestep size is dynamically varied based on the average energy levels ofthe downlink channels, which can be different for different channels.Dynamically adjusting the quantizer step size can reduce the averagemean-squared quantization errors for a given bit rate R of thequantizer. In some implementations, the dynamically adjusted step sizecan also be used to reduce the quantizer rate R without increasing thequantization error.

Information about the dynamically adjusted quantizer step sizes iscontained in side information that a CU sends to the RUs. The RUs canreconstruct the quantized frequency-domain IQ symbols based on the stepsize information. In some implementations, the CU sends some sideinformation to the RUs once per subframe, and the other side informationonce per-REG or once per-PRB. At the beginning of each subframe, the CUsends side information that contains information about the position ofthe CS-RS and the CSI-RS, the step size associated with the CS-RS andthe CSI-RS, and the length of the control region. In someimplementations, the information about the actual step size of thequantizer is sent before each REG (in the control region) or beforesending any PDSCH data in each PRB (in the PDSCH region). The PDSCHenergy levels can be different in the 4^(th) OFDM symbol of a time slot.Accordingly, two step sizes can be sent per PRB. The transmission ofside information can be distributed across the subframe evenly to reducethe peak data rate. When each step size is represented by a 12-bitindex, the side information takes less than 5 Mb/s of link capacity.

In some implementations, the same step size is used for both TX antennasof a RU to limit the amount of side information. In otherimplementations, the step sizes for the two TX antennas can bedifferent.

The rate R of the quantizer is chosen so that the quantization noisedoes not impact the UE's receiver performance, including when the mostdemanding (i.e., most noise-sensitive) MCS (modulation and codingscheme) is used in PDSCH. In some implementations, a 9-bit or 10-bitquantizer delivers an SQNR (signal-to-quantization noise ratio) in therange of 50-60 dB, which is more than 20 dB higher than the target SINR(signal-to-interference-plus-noise ratio) required for uncoded 64-QAM. Aquantizer rate of 9-10 bits can produce a maximum data rate of 302-336Mb/s, which represents a 17-25% compression relative to the maximum datarate in Method I.

III. Adaptive Rate and Step Size Quantization

In a third compression method, both the rate R and the step size Δ ofthe quantizer are dynamically adjusted based on the quantization noisetolerance of each downlink channel. Dynamically varying the quantizerrate R can reduce the average data rate but does not reduce the peakdata rate, and the reduced average data rate can reduce the averagepacket length and the latencies at the Ethernet switches.

The relationship between the quantizer rate R and the performance of thedownlink channel is explained below using an example scenario where eachUE has one RX antenna and each RU has one TX antenna. The discussionsand the calculations can be readily extended to UEs and RUs that havemore than one antenna. In the example, the frequency-domain IQ symbol rreceived by the UE can be written as:

r=(s+q)×h+i+w,

where s represents a complex-valued frequency-domain IQ symbol having anaverage energy E_(s), h is the corresponding complex-valuedfrequency-domain channel gain, q is the quantization noise, and i and wrepresent the received interference and thermal noise, respectively. Thesignal-to-quantization noise ratio of the quantizer, SQNR, is defined tobe E_(s)/E_(q), where E_(q) is the average energy of the quantizationnoise.

The signal to interference plus noise ratio received at the UE isdenoted as SINR′ and can be written as:

$\begin{matrix}{{{SIN}R^{\prime}} = {E_{s} \times {❘h❘}^{2}/( {E_{i} + E_{w} + {E_{q}{❘h❘}^{2}}} )}} \\{= {{SIN}R/( {1 + {{SIN}R/{SQNR}}} )}}\end{matrix},$

where SINR=E_(s)×|h|²/(E_(i)+E_(w)) is the SINR received at the UE inthe absence of any quantization noise, E_(i) is the energy of theinterference noise, and E_(w) is the energy of the thermal noise. Basedon the equation for SINR′, when SQNR>>SINR, SINR′≈SINR. In other words,the quantization noise does not have a substantial or noticeable impacton the performance of the signal received at the UE when SQNR>>SINR.

The SQNR increases with the quantizer rate R, e.g., by about 6 dB forevery increment of R by 1 when R is large. If SINR_(target) representsthe desired SINR required at the UE for a given MCS (modulation andcoding scheme) for reliable reception, implementing the quantizationdoes not cause SINR′ to drop noticeably below the SINR_(target) when thequantizer rate R is chosen such that SQNR>>SINR_(target). Accordingly,when the target SINR for a modulation format is low, the rate R (i.e.,the accuracy) of the quantizer can be reduced.

In some implementations, the quantizer rate R for PDSCH transmissionwill be the highest for PDSCH MCS of 28 and will be the lowest for PDSCHMCS of 0, which respectively correspond to the most and least demanding(in terms of noise sensitivity) modulation and coding schemes currentlysupported in the LTE standard. In the control channels, the underlyingmodulation format is QPSK/BPSK and a relatively low quantizer rate R canbe used. In some implementations, when a relatively low quantizer rateis used, the SINR received at UEs having good channel conditions can bereduced by the quantization noise. However, the reduced SINR does notsubstantially affect the performance of the UE when the reduced SINR isabove the target SINR.

Similar to Method II, the CU sends side information that containsinformation about the step size of the quantizer to the RUs to help theRUs reconstruct the frequency-domain IQ symbols from the received databits. In addition, the CU also dynamically sends the quantizer rate R tothe RUs for each REG and PRB and for the reference signals CS-RS andCSI-RS. Dynamically varying the quantizer rate and step size can reducethe quantization noise caused by a fixed average quantizer rate.Alternatively, when a certain average amount of quantization noise ispermissible for the signal transmissions, the average quantizer rate canbe reduced when the quantizer rate is dynamically adjusted instead ofbeing fixed.

In addition to compressing the symbols being sent to the RUs, the CU canfurther reduce the average data rate between the CU and the RUs by notsending any data for unused REGs or PRBs. For example, when only 50% ofthe REGs and PRBs in a time slot are in use, e.g., carrying data, theaverage data rate is further reduced by 50%.

When multiple TX antennas are used, the same quantizer rate and stepsize can be used for all antennas of each RU so that the amount of sideinformation does not grow with the number of TX antennas. In someimplementations, the quantizer rate and the step size can be differentfor each antenna and the average quantizer rate is then further reduced.

In the description of the quantizers in Methods I-III, we have assumed ascalar uniform quantizer, because of its ease of implementation.However, these methods are equally applicable to other types ofquantizers, such as non-uniform scalar quantizers, vector quantizers,etc. We varied the step size and the rate of the quantizer to adapt thequantizer to the characteristics of the quantized symbols. It is alsopossible to vary other parameters of the quantization process, such asthe gain of the quantizer input.

IV. Modulation-Level Compression

In this fourth compression method, the CU sends the frequency-domain IQsymbols in the form of binary data based on the structure of thefrequency-domain IQ symbols known to the CU and without implementing anyquantization. As discussed previously, the frequency-domain IQ symbolsbelong to a discrete-amplitude signal constellation, or they can bederived by transforming modulation symbols chosen from adiscrete-amplitude signal constellation. By sending binary datarepresenting the discrete-amplitude signals along with side informationrequired to apply any required transformations, the controller can avoidquantization noise.

In use, the CU sends the binary data representing the modulation symbolsto the RUs one OFDM symbol at a time in the same order as the symbolsare to be transmitted by the RUs over the air. In particular, the binarydata that represents the control channels is sent in groups of REGs, andthe binary data that represents the shared data channels is sent ingroups of 12-symbol blocks that belong to the same PRB. Furthermore, atthe beginning of each time slot, the CU sends some portions of sideinformation to the RUs. Other portions of the side information are sentat the beginning of each REG in the control region and before sendingthe data in the first PDSCH OFDM symbol of that time slot. The RUs parsethe received data and reconstruct the frequency-domain IQ symbols basedon the side information.

In this method, some of the baseband modem transmitter functions areimplemented in the CU and some other baseband modem transmitterfunctions are implemented in the RUs. For example, the forward-errorcorrection function is implemented in the CU, whereas the precoding andthe IFFT functions are implemented in the RUs. The downlink processingcan be partitioned between the CU and the RU in many other ways. It iseven possible to move the entire downlink modem processing to the RU. Inthis case the controller sends all necessary data, including thetransport block data, to the RU along with all necessary sideinformation. This will reduce, e.g., minimize, the data rate between thecontroller and the RUs, but may increase the amount of processing in theRUs. In some cases, the interface between the controller and the RUs isimplemented using a so-called FAPI (Femto Application PlatformInterface) developed by the Small Cell Forum, except that the FAPI willbe implemented over an Ethernet network.

Below, we discuss the representation of frequency-domain IQ symbols bybinary data for each type of downlink channel.

(i) CS-RS Reference Symbols

The CS-RS reference symbols are complex-valued binary symbols chosenfrom a QPSK constellation, whose gain may remain constant during thesubframe. When each RU has multiple TX antennas, the CS-RS referencesymbols also include “zero” symbols to avoid interference between theantennas. The CS-RS reference symbols on different antennas differ onlyin their relative positions on the resource grid (see, e.g., grids 730,740 of FIG. 7 ). The CU includes in the side information a 3-bit indexto represent the CS-RS frequency shift and a 12-bit number to representthe gain. The side information is sent to the RUs at the beginning ofeach subframe, through which the RUs learn about the positions of allCS-RS reference symbols in the resource grid for all TX antennas, exceptfor a fixed frequency index offset between 0 and 5. The frequency indexoffset depends on the Cell-ID. Based on the frequency index offset, theRUs can determine the position of the zero REs, for which no data bitsneed to be sent. For the nonzero CS-RS REs, two bits are sufficient torepresent each CS-RS symbol. The RUs receiving the binary data, two bitsfor each RE, can reconstruct the IQ symbol by inserting the correctcomplex-valued CS-RS symbols and the “zero” REs into the resource gridfor each TX antenna based on the side information.

(ii) CSI-RS Reference Symbols

The CU can handle the CSI-RS symbols used in Transmission Mode 9 ofRelease 10 similarly to the CS-RS reference symbols discussed in section(i). At the beginning of each subframe, the CU sends to the RUs sideinformation to indicate the position of the CSI-RS symbols in theresource grid. The side information can be based on parameters such asCSI configuration, ZeroPower CSI-RS Index, scale factor, etc. Using theside information and the data received from the CU, which is two bitsfor each RE, the RUs can insert the correct complex-valued CSI-RSsymbols and the “zero” REs into the resource grid for each TX antenna.

(iii) Control Symbols

The frequency-domain IQ symbols in the control region (i.e., thedesignated first 1-3 OFDM symbols) that are not used by CS-RS belong toPCIFCH, PHICH or PDCCH. In some implementations, the control symbols arerepresented by binary data on a per REG basis. Each REG has 4 REs thatare contiguous, except for the CS-RS reference symbols inserted inbetween. Each control channel is carried in multiple REGs that arespread in frequency (i.e., the REGs are located in different parts ofthe transmission frequency band). For each REG, the CU sends sideinformation to the RU for the RU to parse the received binary data. Theside information is sent per REG and may include 2-bit data to representthe channel type (e.g., PDCCH, PCFICH, PHICH, or unused) and 12-bit datato represent channel gain. At the beginning of each subframe, the CUsends to the RU 2-bit side information to indicate a length of thecontrol region. In some implementations, to process the receivedsignals, the RUs do not need to know in advance the location of thedifferent control channels in the control region.

When each RU has multiple TX antennas (e.g., N TX antennas, where N isan integer larger than 1), the radio network transmits the controlsymbols using Alamouti TX diversity. In implementations, the CU sendsthe 16-bit binary data that represents the 4 QPSK (quadrature phaseshift keying) symbols in each REG to the RU. The RU implements signchange and conjugation operations for TX diversity to generate the 4×NQPSK symbols that represent the frequency-domain IQ symbols in the REGfor all N TX antennas.

The PHICH can be represented by binary data based on the fact that thetransmitted frequency-domain symbols for PHICH are also chosen from adiscrete signal constellation. Each PHICH represents 1-bit of ACK/NAK(acknowledgement/negative acknowledgement) information for uplink HARQ(hybrid automatic repeat request). The PHICH bit is encoded into acomplex-valued 12-symbol sequence chosen from a binary BPSK signalconstellation with a 45 degree rotation. The CU can transmit binary datarepresenting up to 8 PHICH bits together in a PHICH group. For thetransmission, the complex-valued symbols representing all PHICH bits inthe PHICH group are summed together to obtain 12 complex-valued PHICHgroup symbols. As can be seen these symbols are chosen from adiscrete-amplitude constellation. The real and imaginary components ofthe 12 complex-valued PHICH group symbols can each be represented by aninteger in the interval [−6, 6], together with a gain that may remainconstant during the subframe. The 12 complex-valued PHICH group symbolsare mapped to 3 REGs, e.g., in the first OFDM symbol of the controlregion after applying the TX diversity on a per REG basis. The CU sendsto the RUs a gain value represented by a 12-bit index, followed by 8-bitdata that represents the real and imaginary components of eachcomplex-valued PHICH group symbol before applying the TX diversity. TheRUs can use the received information to apply the TX diversity andreconstruct the frequency-domain IQ symbols for all TX antennas.

In some implementations, the PHICH symbols can also be transmitted usinga 16-bit representation of the real and imaginary components of thefrequency-domain IQ symbols for each antenna. Compared to the 8-bitrepresentation, the data rate between the CU and the RUs for the 16-bitrepresentation is higher; however, the RUs can reconstruct thefrequency-domain IQ symbols in a simpler way.

PCFICH and PDCCH can also be readily represented by binary data andtransmitted from the CU to the RUs. In particular, each REG for PCFICHor PDCCH carries 4 QPSK symbols, which are sent on multiple TX antennasusing Alamouti TX diversity. The CU sends 2 bits of data per RE, or 8bits of data per REG to the RUs, which represent the modulated symbolsbefore TX diversity.

(iv) PDSCH Symbols

Most of the REs in the OFDM symbols that are outside the control regionare used by PDSCH, except that the PBCH uses the middle 72 subcarriersin the first 4 OFDM symbols in the first time slot of every 10 ms radioframe, and that the PSS/SSS uses the middle 72 subcarriers in the last 2OFDM symbols in time slots 0 and 10 of every 10 ms radio frame. ThePDSCH symbols for single-antenna transmission are complex-valued and arechosen from a QPSK, 16-QAM or 64-QAM constellation, which can berepresented by 2, 4 or 6 bits of data, respectively. The gain of a givenPDSCH symbol may remain constant during the subframe (except possibly inthe 4^(th) OFDM symbol of each time slot), and the gain for differentPDSCH channels can be different. Resources assigned to each PDSCH are inone or more consecutive VRBs (virtual resource blocks) and can be mappedto PRBs in a localized (consecutive) or distributed (non-consecutive)manner. In some implementations, the CU assumes that the PDSCH changesat every PRB boundary, and sends side information to the RUs on a perPRB basis. The update of side information on a per PRB basis cansimplify the operation of the RUs in reconstructing the PDSCH symbols.In other implementations, localized resource allocation is used and theCU sends side information on a per channel basis, which is less frequentthan sending the side information on a per PRB basis.

For the purpose of discussion, we assume that the CU sends the per-PRBside information before sending the first OFDM symbol of the time slotoutside the control region. The side information includes a 1-bit indexthat indicates whether or not PDSCH symbols are present for transmissionand another 1-bit index that indicates the presence of PSS/SSS ineven-numbered time slots or the presence of PBCH in odd-numbered timeslots. The side information also includes a 2-bit index that representsthe modulation order (BPSK for DM-RS, QPSK, 16-QAM or 64-QAM), a 4-bitindex that represents the PDSCH transmission mode (e.g., FIG. 8 , TM#1-9), and an index representing the precoding coefficients or a 16-bitrepresentation of each complex-valued precoding coefficient (TM #9). Theside information is followed by binary data representing the PDSCHmodulation symbols.

The RUs use the side information to complete the baseband modemoperations and generate the frequency-domain IQ symbols. In theimplementations where the PDSCH uses Transmission Mode 9, thedemodulation reference symbols (DM-RS) can also be viewed as QPSKsymbols using the same gain as the PDSCH symbols. Accordingly, nospecial treatment may be required for the REs of DM-RS.

In the previously discussed example in which a radio network implementsthe 10 MHz FDD LTE, there are 50 PRBs in each 0.5 ms time slot. EachOFDM symbol that carries no CS-RS has 12 PDSCH REs in each PRB, whereasthe OFDM symbols that carry CS-RS have 8 PDSCH REs per PRB (assumingthat there are 2 TX antennas). A PRB that carries PBCH has 32 REs forthe PDSCH.

When multiple antennas are in use for a PDSCH, the CU can reduce theamount of data that needs to be sent to the RUs based on the knowledgeof the underlying structure of the multiple-antenna transmitter. Thefrequency-domain IQ symbols in TX diversity are chosen from a QAMconstellation, and at least some of these IQ symbols are dependent oneach other. For example, a group of N² frequency-domain IQ symbolstransmitted on N TX antennas can be derived from N input modulationsymbols, which are chosen from a discrete-amplitude complex-valuedconstellation, using operations such as sign changes or complexconjugations. Accordingly, instead of sending information for N×N=N²frequency-domain IQ symbols, the CU can send information for the N inputmodulation symbols and indicate that TX diversity is used. The RUs canimplement the TX diversity operations to produce the N² symbols fortransmission in N subcarriers on N TX antennas. As a result, the datarate between the CU and the RUs does not increase when the number of TXantennas is increased.

In general, the frequency-domain IQ symbols for an N-antenna MIMOtransmitter can be written as:

Y=PX,

-   -   where X is a K-dimensional PDSCH input vector whose components        are chosen from the underlying QAM signal constellation, P is an        N×K precoding matrix, and K is the number of layers being        transmitted. Instead of quantizing Y as if it were some        continuous random vector, the CU sends data bits that represent        the K modulation symbols in the vector X along with the        precoding matrix. The precoding matrix does not vary within a        subframe, and, in some implementations, the CU only sends the        precoding matrix once per PRB instead of once every OFDM symbol.

For Release 8 closed-loop MIMO, the precoding matrix is chosen from afixed set and the precoding matrix can be represented by a shortprecoding index of a few bits. In the transmission Mode 9 of Release 10,less than 64 bits are needed to represent the precoder coefficients (16bits per complex coefficient) (assuming that there are 2 TX antennas).

The data rate for the frequency-domain IQ symbols can be significantlyreduced when the number of layers K is less than the number of the TXantennas N. The data rate increases with the number of layers. However,even when K=N (i.e., full-rank spatial multiplexing), sending binarydata representing the QAM modulation symbols instead of sending theprecoded frequency-domain IQ symbols can reduce the data rate and avoidquantization noise. To transmit K layers, the data rate for the PDSCHinput data is K times the data rate for a single-layer.

(v) Other Symbols

The CU can readily handle the binary representation of symbols on theother downlink common channels. For example, PBCH REs can be treatedsimilarly to PDSCH using QPSK modulation and TX diversity. The CU canuse 1 bit of side information to indicate the presence or the absence ofthe PBCH in odd time slots. In some implementations, the REs that carrythe synchronization symbols PSS/SSS are sent without any compression as16-bit integers to represent the real and imaginary components of thefrequency-domain IQ symbols. Similarly, 1 bit of side information can beused to indicate the presence or the absence of PSS/SSS in even timeslots.

(vi) Summary

In the method we described above, the downlink baseband modem functionsare split between the CU and RUs in such a way that reduces the datarate on the Ethernet network, while keeping the processing complexityvery low at the RUs. For example, using the specific partitioningdescribed above, the bit rate on the Ethernet network can be reduced toaround 100 Mb/s for two transmit antennas and two layer PDSCHtransmission. Actual data rate will be even lower when the airlinkresources are not 100% utilized. In addition to a lower bit rate, themethod also eliminates quantization noise altogether. Other ways ofpartitioning the data between the CU and the RUs are possible. Forexample, it is possible for the RUs to perform all the physical layerfunctions, while the scheduling and higher-layer processing is performedin the CU.

2. Uplink Compression within a Cell

A. General Description of the Uplink Compression

The LTE uplink in the radio network of this disclosure is different fromthe downlink in many ways. For example, the uplink signals received bydifferent RUs in the same cell are not identical. The different uplinksignals can have different channel gains, noise and interference levelswhich can be exploited by the controller for power and diversity gains.However, when a cell contains multiple RUs and all RUs send theirreceived signals to the CU, the CU receives a larger amount data on theuplink than it broadcasts to the RUs on the downlink.

Similar to the techniques used in downlink compression, the techniquesfor uplink compression also take into account one or more of thefollowing additional differences between uplink and downlink. First, onthe uplink, without full-blown demodulation and decoding, the RUs cannotknow precisely the discrete-amplitude modulation symbols transmitted bythe UEs.

Second, the modulation format on the LTE uplink, SC-FDMA (single carrierfrequency division multiple access), is different from the OFDMA schemeused on the downlink. Instead of using the modulated symbols or theirprecoded versions as frequency-domain IQ symbols, the modulation symbolsin SC-FDMA are time-domain signals. These time-domain signals aretransformed by the UE into frequency-domain IQ symbols using a DFT(Discrete Fourier Transform). Compared to the symbols on the downlink,the frequency-domain IQ symbols obtained from the DFT transformation canexhibit a less uniform and more like a truncated Gaussian statistics,especially when the UE is assigned many RBs.

On the uplink, resources in a PRB are allocated on a contiguous manner,and frequency hopping may be utilized between two time slots of asubframe. As an example, the PUSCH PRBs (with DM-RS in the middle)assigned to a UE are consecutive and can hop between slots 0 and 1 witha known gap between them. The 4^(th) OFDM symbol of each assigned PUSCHPRB is DM-RS. The SRS, if present, is transmitted in the last symbol ofthe subframe, e.g., at every other subcarrier. The PUCCH transmissionsinclude QPSK symbols modulating a complex-phase sequence and anorthogonal cover transmitted over two PRBs at the opposite edges of aband. In some implementations, multiple UEs can transmit PUCCH signalson the same PRBs in the same subframe. The first L (which is an integer)PRB pairs carry CQI/PMPRI transmissions, possibly together with HARQACK/NAKs, using Format 2. Additional PRB pairs are available for HARQACK/NAKs and scheduling requests.

Referring to FIG. 8 , for PUSCH transmission, a UE 1204 modulates 1200and transforms 1210 time-domain symbols×1202 into frequency-domainsymbols s 1203, performs a resource mapping 1212, and then performs afull IFFT 1214 to generate the time-domain signals for transmission overthe air to the RUs. One or more RUs 1206 in a cell receive thetransmitted signals through one or more channels 1208 via its antennas,apply RF processing to generate the received time-domain IQ signals, andapply an FFT 1220, 1222 to produce the received frequency-domain IQsignals r₁ 1216, r₂ 1218.

Assuming that a cell includes K RUs, where K is a positive integer, andthat the kth RU has two antennas for receiving signals (RX antennas)from a UE that has one TX antenna for transmitting the signals, thefrequency-domain IQ symbol, r_(ki), received at the l'th RX antenna (l=1or 2) of kth RU in some fixed frequency position in an OFDM symbol canbe expressed in the following forms:

r _(k1) =s×h _(k1) +i _(k1) +w _(k1),

r _(k2) =s×h _(k2) +i _(k2) +w _(k2),

-   -   where s is the frequency-domain IQ symbol transmitted by the UE        (see, e.g., FIG. 8 ), h_(k1) and h_(k2) are the channel        coefficients, i_(k1) and i_(k2) represent interference from UEs        in other cells, w_(k1) and w_(k2) are thermal noise,        respectively for the two RX antennas.

The total energy levels of the received symbols r_(k1) and r_(k2) at thekth RU are:

E _(t,k1) =E _(s) ×|h _(k1)|² +E _(i,k1) +E _(w,k1),

-   -   where l=1, 2, E_(s)×|h_(k1)|², E_(i,k1) and E_(w,k1) represent        the average energy of the received symbols and the average        energy of the interference and noise received via the lth        receive antenna of the kth RU, respectively. Generally, the        average energies of the received symbols, E_(s)|h_(k1)|², are        different on different uplink channels because the required SINR        at these channels changes based on the PUCCH Format (Format 1,        1a, 1b, 2, 2a, 2b) and the PUSCH MCS (e.g., QPSK or 64-QAM). The        interference energy, which is caused by other UE transmissions        in nearby cells, can also vary among different PRBs, which can        cause additional variations in the energy levels of the received        symbols at the RUs.

The RUs implement the uplink compression using a quantizer to reduce thedata rate of transmissions from the RUs to the CUs. For the purpose ofdiscussion, we assume that the quantizer is a uniform scalar quantizerhaving a rate R_(k1) and a step size Δ_(k1) and quantizes the real andimaginary components of the received frequency-domain IQ symbolsindependently at the lth antenna of the kth RU. Other quantizationtechniques, such as non-uniform scalar quantization or vectorquantization, can also be used with the techniques discussed below.

Referring to FIG. 9 , the RU 1300 sends the bits 1302 that represent anoutput of the quantizer 1304 to the CU 1306 in Ethernet frames throughan Ethernet network 1308. The CU 1306 reconstructs a quantized versionr_(k1)′ of each received symbol r_(k1):

r _(k1′) =s×h _(k1) +i _(k1) +w _(k1) +q _(k1),

-   -   where q_(k1) is the complex-valued quantization noise having an        average energy E_(q,k1). The performance of the quantizer 1304        can be measured by its signal-to-quantization noise ratio        (SQNR), which is defined as:

SQNR_(k1) =E _(t,k1) /E _(q,k1),

-   -   where E_(q,k1)=2×MSE and MSE is the mean-squared error of the        uniform scalar quantizer.

The quantized symbols are sent to the CU through the Ethernet network.In some implementations, the rate R_(k1) of the quantizer is chosen sothat the quantization noise does not substantially affect theperformance of the receivers at the CU. In the absence of quantizationnoise and assuming that the noise and interference received on all theantennas are uncorrelated, the performance of a receiver at the CU forMRC (a maximum-ratio combiner) can be represented by the effective SINR:SINR=Σ_(k) (SINR_(k1)+SINR_(k2)),

-   -   where SINR_(k1)=E_(s)×|h_(k)|²/(E_(i,k1)+E_(w,k1)) is the SINR        on the lth RX antenna of the kth RU.

When the interference i_(k1) on different RX antennas is correlated, theCU receiving the compressed symbols from the RUs can apply IRC(interference rejection combining)

The performance of the IRC is determined based on the sum of the SINRson all RX antennas as shown by the above equation, except that each SINRfor a given RX antenna includes the effect of the spatial whiteningfilter.

Next, the effect of non-zero quantization noise on the performance ofthe receivers at the CU is considered. Thesignal-to-interference-plus-noise-plus-quantization noise-ratio at theoutput of the MRC receiver in the CU, SINR′, is:

${{{SIN}R^{\prime}} = {{\sum}_{k}( {{{SIN}R_{k1}^{\prime}} + {{SIN}R_{k2}^{\prime}}} )}},\begin{matrix}{{{where}{SIN}R_{k1}^{\prime}} = {E_{s} \times {❘h_{k1}❘}^{2}/( {E_{i,{k1}} + E_{w,{k1}} + E_{q,{k1}}} )}} \\{= {{SIN}R_{k1}/( {1 + {( {1 + {{SIN}R_{k1}}} )/{SQNR}_{k1}}} )}}\end{matrix}$

In other words, the SINR′ is the sum of thesignal-to-interference-plus-noise-plus-quantization noise ratios at eachbranch of the MRC that receives quantized symbols from respectiveantennas in the cell. If the quantizer rates R_(k1) are chosen for allantennas (for all k and l) such that:

SQNR_(k1)>>1+SINR_(k1),

-   -   then SINR_(k1′)≈SINR_(k1), and SINR′ approximately equals the        ideal SINR with no quantization noise, i.e., SINR′=SINR=Σ_(k)        (SINR_(k1)+SINR_(k2)).

The amount of degradation caused by the non-zero quantization noise inthe effective SINR_(k1) for each antenna of the RU can also bedetermined using the above formula. The amount can be calculated asSINR_(k1)/SINR_(k1)′, which can be written as a function ofSQNR_(k1)/(1+SINR_(k1)).

Table 1 shows the amount of degradation in SINR_(k1) per RX antenna dueto the quantization noise as a function of the ratioSQNR_(k1)/(1+SINR_(k1)). The data illustrates that when the SQNR_(k1) isat least 20 dB above 1+SINR_(k1), the reduction in SINR_(k1) due to thequantization noise is less than 0.05 dB.

TABLE 1 Reduction in SINR_(kl) due to Quantization Noise SQNR/(1 + SINR)SINR/SINR′ (dB) (dB) 0 3.01 5 1.19 10 0.41 15 0.14 20 0.04 25 0.01

B. Quantization Methods

Below, four different quantization methods for uplink compression aredescribed, with an increasing compression ratio from Method I to MethodIV.

I. Fixed Quantization

In this method, a fixed uniform scalar quantizer having a fixed rateR_(k1)=R₀ and a fixed step size Δ_(k1)=Δ₀ is used. As an example, R₀=12and the quantized IQ stream is sent from a RU to the CU at a total bitrate of about 403 Mb/s for two RX antennas of the RU. Accordingly, thefixed quantizer having a step size of 12 bits can be implemented withouta high level of complication and without substantially affecting theperformance of the signal transmission. The data rate of 403 Mb/sbetween the CU and the RUs is relatively high. When K RUs are sendingquantized frequency-domain IQ symbols at a data rate of 403 Mbps towardsthe CU for the same OFDM symbol, the aggregate bit rate between thenearest Ethernet switch and the CU is K×403 Mb/s, which can berelatively high for large K.

II. Adaptive Step-Size Quantization

In this method, the quantization is implemented using a uniform scalarquantizer that has a fixed rate R_(k1)=R₀, and a step size Δ_(k1) thatis adjusted dynamically. In one implementation, the step size may beupdated on a per-PRB basis and independently for each antenna. For eachOFDM symbol, the step sizes Δ_(k1) are individually varied for eachuplink channel that uses resources on that OFDM symbol. For example,Δ_(k1) can be selected based on the average energy of thefrequency-domain IQ symbols received in each uplink channel. In someimplementations, the average energy of the IQ symbols on a given channelis estimated using the symbols to be quantized at the RUs. The step sizeof the quantizer can then be adjusted based on an assumed distributionof those symbols to be quantized. In some implementations, thedistribution is determined based on the size of the DFT used by the UE.Optimizing the step size dynamically and independently for each channelcan allow signals to be transmitted from the RUs to the CU at a higherSQNR at the same data rate. In addition, optimizing the step sizedynamically and independently for each channel can be used to lower thedata rate without reducing the SQNR.

In some implementations, it may not be necessary to vary the quantizerstep size Δ_(k1) in every OFDM symbol, e.g., when the average energy ofa symbol received by the RU from a UE does not vary significantly withina subframe. In such implementations, the step size for the first OFDMsymbol is determined using the received IQ symbols in the first OFDMsymbol, e.g., to avoid delay. When the number of symbols available isinsufficient to accurately estimate the average energy in the first OFDMsymbol, the average energy estimate and the step size can be refined insubsequent OFDM symbols.

The quantizer rate R.sub.0 is chosen to be high enough so that theperformance of the receiver at the CU does not degrade for the highestMCS. For example, when R₀=10, the SQNR of the quantizer is about 52 dB(assuming a Gaussian input), which is more than 20 dB higher than theminimum SINR required for reliable communications at the highest uplinkMCS.

As shown in Method I, an SQNR that is 20 dB above the minimum requiredSINR allows the receiver at the CU to operate with a performancedegradation due to quantization of no more than 0.05 dB. A quantizerrate R₀ of 10 can produce an IQ data rate of about 336 Mb/s for two RXantennas of a RU. This represents a compression ratio of 10/12, or is17% higher compared to the compression rate of Method I. Because thequantizer rate R₀ is fixed, all frequency-domain IQ symbols received bythe RUs, including IQ symbols that carry no information, are quantizedand sent to the CU. When an optimized step size is used, the value ofthe quantizer rate required to achieve a desired SQNR is lower than whenthe step size is not optimized.

The RUs use different step sizes for different PUSCH/PUCCH/SRS/PRACHchannels based on information about the uplink channel boundariesreceived from the CU. In some implementations, the uplink channelboundaries for each PRB are indicated by downlink side information sentby the CU to the RUs. Referring to FIG. 10 , the side information 1404for use in an uplink (UL) subframe N is sent by the CU 1400 in thedownlink (DL) subframe N-4 (1406) to the RUs 1402.

Examples of the downlink side information 1404, e.g., the contents andsizes, are as follows. The PUSCH or PUCCH PRBs assigned to the samechannel are consecutive, and the channel boundaries for PUSCH and PUCCHcan be indicated by a 6-bit position index and a 6-bit length field. TheCU can also send indications of the channel type (e.g., PUSCH, PUCCH,SRS, PRACH, etc.) to the RUs using a 2-bit index to facilitate the RUsto model the statistical distribution of the received symbols.Furthermore, one bit of the side information can be used to indicate thepresence of the SRS (sounding reference signal), which can occupy thelast OFDM symbol of the subframe. Also, the position of the PRACH, whenpresent, can be indicated by a 6-bit index.

Based on the knowledge of the PUSCH/PUCCH channel boundaries, the RUsdetermine for each OFDM symbol the average energy of the receivedfrequency-domain IQ symbols that belong to the same channel (or from thesame UE). The RUs then choose the step size Δ_(k1) of the quantizerbased on the determined average energy. In some implementations, a RUdetermines the optimum step size without distinguishing the differentchannel types (i.e., PUSCH or PUCCH). In some implementations, a RU usesthe downlink side information about the channel type to facilitatechoosing the optimum step size without any measurement related to thereceived frequency-domain IQ symbols (e.g., average energy). For theSRS, the RUs can estimate the average energy across the entire OFDMsymbol and determine the optimum step size. Alternatively, the RUs cansplit the subcarriers in an OFDM symbol that carries SRS into subbandsand optimize the step size for each subband. In some implementations, afixed pre-determined step size may be used to quantize the SRS signal.For the PRACH, the step size can be determined based on the peak powervalue of the received signal, or it may be fixed.

The RU may implement the uniform scalar quantization with variable stepsizes by applying a gain γ_(k1) to normalize the energy of the IQsymbols to be quantized. The RUs then quantize the real and imaginarycomponents of the IQ symbols using a uniform scalar quantizer having afixed step size Δ_(k1)=Δ₁. In some implementations, the real andimaginary components are symmetric, and the same gain and scalarquantizer can be used for both the real and the imaginary components.

The RUs send uplink side information about the selected step sizes tothe CU, along with the data bits representing the quantizedfrequency-domain IQ symbols, based on which the CU reconstructs thereceived IQ symbols.

In some implementations, each step size of the quantizer is representedby a 12-bit index in the side information. In some implementations, theRUs update the step size in every OFDM symbol, which can increase theamount of side information transmitted in one time slot by up to afactor of 7. For the SRS, the RUs send to the CU the uplink sideinformation about the step size for each subband before sending thedata. For the PRACH, the information about the step size can be sentbefore the quantized PRACH data is sent.

III. Adaptive Rate and Step Size Quantization

In this method, in addition to dynamically adjusting the step sizeΔ_(k1) of the quantizer, the rate R_(k1) of the quantizer is alsodynamically adjusted for compressing (or quantizing) the IQ stream. Forthe purpose of discussion for this method, PUSCH symbols that carry userdata and PUSCH symbols that carry UCI (uplink control information) arenot distinguished. Also, the same quantizer rate is applied to allsymbols sent by the same UE.

The quantizer rate can be dynamically adjusted, for example on a per PRBbasis. As discussed previously, for PRBs that carry PUSCH IQ symbolsfrom a relatively low MCS, a lower quantizer rate can be used than therate for the PRBs carrying PUSCH IQ symbols from a relatively high MCS.Similarly, some PRBs carrying PUCCH symbols can be quantized at arelatively low rate. The SINR required for these PRBs (for PUSCH orPUCCH) to provide a reliable reception at the CU can be relatively low.Accordingly, these PRBs can tolerate a relatively high level ofquantization noise. Furthermore, those PRBs not carrying any data do notneed to be quantized. The high tolerance of quantization noise and thereduced number of PRBs to be quantized on the uplink can savetransmission bandwidth between the RUs and the CU. Adjusting thequantizer rate based on these considerations can reduce the average datarate on the uplink.

As discussed previously, the quantizer rate for each antenna of the RUis chosen to be relatively high such that SQNR_(k1)>>1+SINR_(k1), whereSQNR_(k1) is the quantizer SQNR and SINR_(k1) is the receiver SINR forthe lth antenna of the kth RU of a cell. When such a relationshipbetween the SQNR_(k1) and the SINR_(k1) is satisfied, the quantizationnoise is much lower than the interference plus noise seen on the antenna(lth antenna of the kth RU), and the effect of the quantization onSINR_(k1) is small.

In some implementations, a RU does not determine the SINR_(k1) on itsown. Instead, the RU learns from the CU the target SINR, SINR_(target),across all antennas of the cell. The SINR_(target) is a function of theMCS used in each PRB. The CU uses the power control loop to drive thetransmit powers of a UE to a baseline level, and the UE adjusts thebaseline transmit power according to the MCS used in a given PRB so thatthe SINR in the eNodeB is approximately equal to the SINR_(target).

In some implementations, the RUs choose the quantizer rate such that thequantization noise does not substantially reduce the SINR at thereceiver of the CU to below the target SINR. When the CU controls thetransmission power of the UE by accurately tracking channel changes, theSINR at the receiver of the CU approximately equals SINR_(target).Furthermore, when SQNR_(k1)>>SINR_(target)>SINR_(k1),SINR′=Σ_(k)(SINR_(k1)′+SINR_(k2)′)≈SINR_(target). In other words, thequantization noise does not substantially reduce the SINR at thereceiver of the CU when the quantizer rate is chosen such thatSQNR>>SINR_(target).

In summary, by selecting the quantizer rate such thatSQNR_(k1)>>SINR_(target), a RU can quantize the IQ symbols withoutproducing quantization noise that substantially affects the performanceof the CU receiver or prevents reliable communication between the CU andthe RU.

In the techniques describe above, for a given PRB, the RUs in the samecell use the same quantizer rate for all antennas. In someimplementations, the SINRs of different antennas (SINR_(k1)) can besignificantly different. In such implementations, different quantizerrates can be chosen for antennas having different SINR_(k1) in the samecell. For example, the quantizer rates can be chosen so that SQNR isproportional to 1+SINR_(k1). In particular, the quantizer rate for theantenna with a lower SINR_(k1) is chosen to be lower than the quantizerrate for an antenna with a higher SINR_(M). In some implementations,when the SINR_(k1) of some antennas is too low relative to the totalSINR, it is wasteful for the RUs to which those antennas belong totransmit the received IQ symbols to the CU. Significant IQ streamcompression can be achieved when those RUs can determine that thesignals received on their antennas do not contribute significantly tothe overall SINR in the CU and purge or prune the signals (which isequivalent to using a quantizer rate of “0” for these signals).

A RU can adjust the quantizer rate based on the SINR_(k1) seen on eachantenna and additionally, the difference between the SINR_(k1) on itsdifferent antennas and the SINR_(k1) on other antennas in the same cell.In some implementations, the CU selects RUs from which to receivesymbols. The CU can also determine the quantizer rate for each RU basedon past UE transmissions. For example, the CU sets the quantizer rate tobe zero for an antenna when it determines that the SINR_(k1) of thatantenna contributes to less than 5% of the total SINR.

In some implementations, the CU determines the quantizer rate for eachantenna on a per UE basis at the time when the UE transmits a PRACHpreamble. All RUs can be required to forward all PRACH preamble signalsto the CU so that the CU can make an initial determination of theSINR_(k1) for each antenna. The CU can then select the quantizer ratefor each antenna and include this information in the downlink sideinformation it sends to the RUs. The CU is capable of determining thequantizer rate for those RUs from which the CU receives PUSCH or PUCCHsignals transmitted by a UE in a recent subframe. For RUs whosetransmissions for a UE are purged, the CU can determine a quantizer ratebased on the SRS sent by the UE at regular intervals. All RUs can berequired to relay the SRS.

Based on the SRS and the PRACH preamble signals, the CU can determinethe quantizer rate for all RUs in a cell. In addition, the CU canperiodically request the RUs that previously have purged transmissionsfrom the UE to send IQ symbols and use the IQ symbols to update thequantizer rate for those RUs. By adjusting the quantizer rate fordifferent antennas, the average rate of the data sent from the RUs tothe CU can be significantly reduced, especially when there are many RUsin a cell.

In some implementations, purging signals on the PUCCH may be difficultwhen multiple UEs share the same PUCCH resources. In suchimplementations, symbols on the PUCCH are transmitted without purging.The uplink transmission rate is not substantially affected because thePUCCH occupies a variable but relatively small percentage of the uplinkresources. In some implementations, a fixed quantizer rate can be usedfor all antennas on the PRBs assigned to the PUCCH, even when PUCCHtransmissions implement transmit diversity in which the same controlinformation can be sent using different resources. In someimplementations, other, e.g., more sophisticated, quantization andpurging schemes can be used for the PUCCH when the radio network has avery large number (e.g., 16 or larger) of RUs in the cell.

The CU incorporates the quantizer rate for each PRB determined for eachantenna in the downlink side information, which is used by the RUs. Forthose unallocated PRBs that carry no data or for antennas that do notsignificantly contribute to total SINR, the CU sets the quantizer rateto be zero. The side information sent by the CU to the RUs can alsoinclude other information, such as PUSCH MCS and PUCCH Format, and anindex that represents the expected probability distribution of thefrequency-domain IQ symbols in the PRB.

Similar to Method II, the CU sends the side information associated withuplink subframe N in downlink subframe N−4 (see, e.g., FIG. 10 ). TheRUs use the side information received in downlink subframe N−4 to selectthe quantizer step size for each PRB in uplink subframe N. The stepsizes are optimized similar to Method II, i.e., based on the measuredaverage energy and the estimated probability distribution of thereceived IQ symbols. The RUs send the selected step size for eachquantizer to the CU at the beginning of each OFDM symbol beforetransmitting the quantized IQ symbols. Generally, little uplink capacityis used to send the side information for the step sizes.

Quantization based on Method III may not reduce the peak rate of theuplink IQ data compared to Method II. However, the method cansignificantly lower the average bit rate. As an example, the average bitrate can be reduced by more than 50%, when only 50% of the uplinkresources are in use. Adapting the quantizer rate using the techniquesof this method can help reduce the average uplink data rate and the loadon the Ethernet switches.

IV. Predictive Quantization

In the previously described Methods I, II, and III, the signals receivedon different antennas of the same RU are treated as uncorrelated. Inthis fourth method, when the number of receive antennas is greater thanthe number of layers sent by a UE in spatial multiplexing, thecorrelation between signals received on different antennas of the sameRU is used to further reduce the quantizer rate for PUSCH transmissions.In the Release 10 version of the LTE standard, the UE may transmit onmultiple antenna ports. For the purpose of discussion, we assume thatthe UE transmits on the PUSCH using a single transmit antenna port.

As shown previously, signals received by the two antennas of the kth RUin a cell can be represented as:

r _(k1) =s×h _(k1) +i _(k1) +w _(k1),

r _(k2) =s×h _(k2) +i _(k2) +w _(k2).

Furthermore, r_(k2) can be expressed according to the followingpredictor equation:

r _(k2) =a _(k2) ×r _(k1) +z _(k2),

-   -   where the prediction coefficient a_(k2) is given by:

a _(k2) =E{r _(k2) r _(k1) *}/E{|r _(k1)|²},

-   -   and z_(k2) is the prediction error and can be written as:

z _(k2) =r _(k2) −a _(k2) r _(k1).

A RU can estimate the prediction coefficient a_(k2) by calculating theaverage correlation between the signals received at the two antennas,and then dividing the result by the average energy of the signalsreceived on the second antenna. The RU performs the estimation on a perUE basis based on information received from the CU.

Referring to FIG. 11 , the RU first quantizes r_(k1) with a uniformscalar quantizer having a rate R_(k1) and a step size Δ_(k1) to obtainthe first quantized signal r_(k1)′ 1502, where

r _(k1) ′=r _(k1) +q _(k1).

Here q_(k1) is the quantization noise for the symbol received at thefirst antenna. The RU then uses r_(k1)′ to produce 1504 the predictionerror z_(k2)′=r_(k2)−a_(k2)r_(k1)′, which is then quantized with anotheruniform scalar quantizer 1506 having a rate R_(k2) and a step sizeΔ_(k2) to generate the second quantized signal.

z _(k2) ″=r _(k2) −a _(k2) r _(k1) ′+q _(k2).

Here q_(k2) is the quantization noise for the symbol received at thesecond antenna. Bits 1510, 1512 representing the quantized symbolsr_(k1)′ and z_(k1)″ are sent to the CU, along with the predictioncoefficient a_(k2) and the quantizer information R_(k1), R_(k2), Δ_(k1)and Δ_(k2). The CU first reconstructs 1514, 1516 the quantized symbolsr_(k1)′ and z_(k2)′ and then generates the quantized symbol r_(k2)′ 1518according to

r _(k2) ′=z _(k2) ″+a _(k2) ×r _(k1) ′=r _(k2) +q _(k2).

The average energy of the symbol z_(k2)′ is lower than that of r_(k2),and the quantizer rate R_(k2) is generally lower than the quantizer rateused when the RU quantizes r_(k2) directly without prediction. The lowerquantizer rate can reduce the IQ rate.

Again, the SINR in the CU can be written as:

SINR′=Σ_(k)(SINR_(k1)′+SINR_(k2)′),

where

SINR_(k1) ′=E _(s) ×|h _(k1)|²/(E _(i,k1) +E _(w,k1) +E _(q,k1)).

For the first antenna, SINR_(k1)′ can be written as:

SINR_(k1)′=SINR_(k1)/(1+(1+SINR_(k1))/SQNR_(k1)).

Accordingly, when the quantizer rate for the first antenna is chosensuch that SQNR_(k1)>>1+SINR_(k1), the quantization noise does notsubstantially affect SINR_(k1)′.

Similarly, for the second antenna, SINR_(k2)′ can be written as:

SINR_(k2)′=SINR_(k2)/(1+[(1+SINR_(k1))/(1+SINR_(k1))/SQNR_(k2)])).

Here SINR_(k)=SINR_(k1)′+SINR_(k2)′ and is the total SINR in the CU forthe kth RU. Accordingly, when SQNR_(k2)>>(1+SINR_(k))/(1+SINR_(k1)), thequantization noise introduced by the second quantizer does notsubstantially affect SINR_(k2)′.

In some implementations, the two antennas of a RU have the same SINR,i.e., SINR_(k1)=SINR_(k2), and the condition for the quantization noiseto not substantially affect the SINR at the CU can be simplified to:

SQNR_(k2)>>(1+SINR_(k))/(1+0.5×SINR_(k)).

When SINR_(k)>>1, SQNR_(k2)>>2. A uniform scalar quantizer having a rateof about 5-6 can readily satisfy this condition. The resulting IQ ratefor the 2^(nd) antenna is reduced to about 84-101 Mb/s, representing acompression of more than 50%.

To implement the predictive quantization, in some implementations, theCU estimates the prediction coefficients, in addition to determining thequantization rate based on the predictive quantization. The estimatedcoefficients can be sent to the RUs in the downlink side information.Alternatively, the CU can determine the quantizer rate as discussed inMethod III and without relying on predictive quantization. The RUs applythe prediction and send the prediction coefficient as part of the uplinkside information to the CU. In some implementations, the CU determinesthe quantizer rate based on the predictive quantization, and the RUsdetermine the prediction coefficients and send the coefficients to theCU as part of the uplink side information.

V. Uplink Compression of the PRACH Preamble

When an idling UE has data to send or to receive, the UE establishes aconnection with the eNodeB by sending a PRACH preamble to the eNodeB insome designated PRBs that are shared by all the UEs in a cell. In someimplementations, each cell has 64 shared PRACH preamble sequences, someof which are designated for use in contention-free access and the othersare divided into two subsets. In contention-free access, the eNodeBassigns a preamble to the UE. In other situations, the UE selects one ofthe two subsets based on the amount of data to be transmitted. The UEthen randomly picks one of the preamble sequences in the selectedsubset.

A PRACH preamble uses 6 RBs at 1.08 MHz, and the positions of the 6 RBsare determined and signaled to the UEs by the CU. The PRACH preamble canlast 1, 2 or 3 subframes, depending on the length of the cyclic prefixand the guard time. The PRACH opportunities can occur as frequently asonce every 1 ms subframe or as infrequently as once every 20 ms.

In general, the UEs are not scheduled to transmit PUSCH on the PRBsassigned to PRACH. The CU can use non-adaptive HARQ on the uplink toprevent collisions between PRACH and HARQ retransmissions. Thenon-adaptive HARQ changes the RBs used in the transmission for collisionavoidance. The PRACH opportunities can also be chosen to not overlapwith the SRS or the PUCCH transmissions. The UE selects the transmitpower for the PRACH preamble based on open-loop power control where theUE estimates the uplink signal loss based on a measurement of thedownlink signal loss and gradually increases the transmit power afterunsuccessful attempts.

The detection of the PRACH preamble can be implemented partially in theRU and partially in the CU. In some implementations, the RUs know theexact position of the PRACH opportunities and convert the receivedtime-domain IQ symbols (at 15.36 MHz for the 10 MHz FDD LTE standards)into a lower-rate time-domain sequence (e.g., a rate of 1.28 MHz) usinga time-domain frequency shift followed by decimation. The resultingsequence is then converted to frequency domain using an FFT (e.g., a1024-point FFT for the 10 MHz FDD LTE standards). A frequency-domaincorrelation is performed between the FFT output and the frequency-domainrepresentation of the root Zadoff-Chu sequence. The 64 PRACH preamblesequences are derived using a cyclic shift. The complex-valued output ofthe frequency-domain correlator is then converted back to acomplex-valued time domain sequence using an IFFT (e.g., a 1024-pointIFFT).

The RUs and the CU perform the next steps of detecting the PRACHcollaboratively. For example, the RUs can compute a real-valuedtime-domain sequence of 1024 samples by summing the squares of the realand the imaginary components. The RUs can send this information to theCU for further processing. The CU, upon receiving the time-domain powersequence, performs a peak detection to determine the preamble cyclicshift. Such uplink PRACH transmissions are compressed in the time-domainsuch that data compressed in the time-domain is transmitted between theRUs and the CU.

Alternatively, the RUs can send the complex-valued output symbols of theIFFT to the CU and let the CU perform the remainder of the PRACHpreamble detection. In some implementations, the RUs implement the peakdetection, determine the preamble cyclic shift, and send the CU thecyclic shift information. The amount of data transmitted from the RUs tothe CU for PRACH preamble detection is small. In the example of the 10MHz FDD LTE, the amount of data ranges from a few bits to 12-20 Mb/s,depending on whether the real-valued power or the complex-valued IFFToutputs are sent.

In some implementations, when there is no substantial overlap betweenthe PRACH transmissions and other uplink transmissions, no othertransmissions are performed for the RBs that are transmitted on thePRACH.

For the RUs to correctly implement the PRACH preamble detection, the CUcan provide the RUs with configuration information, such as the PRACHconfiguration index, PRACH frequency offset, PRACH Zadoff-Chu rootsequence, etc. The CU can send this information to the RUs when the RUsare initially assigned to the CU or when the PRACH is modified.

The PRACH data may be quantized with a fixed rate quantizer, whose rateis pre-determined by the CU and sent to the RUs when the RUs initiallyconnect to the CU. The quantizer step size may also be fixed, or it maybe dynamically selected by the RUs based on the average energy of thereceived PRACH signal.

Synchronization

In the present disclosure there are some synchronization requirementsthat are generally not applicable to classic base stations.

As explained above, in the present disclosure, some parts of thebaseband processing (e.g., modem functionality) and FFT/RF processing(e.g., radio functionality) are split between a central CU and multipleRUs (RUs) that are connected via a switched Ethernet network (as shownin the figures). In classic base stations, a GPS receiver is typicallyused to acquire time and frequency synchronization and since the modemand RF functions are co-located, they can be synchronized to the GPSreceiver. In the present disclosure, in some implementations, a GPSreceiver is only available in the CU, and is not available in the RUs tokeep the system cost low and to avoid the installation complexity. TheCU can also acquire timing and frequency synchronization through othermeans, for example from a network server or by listening to signalstransmitted by a macro cell base station nearby. In someimplementations, a timing transport protocol is used to carry a stableabsolute timing phase and frequency reference that is traceable tocoordinated universal time (UTC/GPS) from the CU to the RUs. The timingtransport protocol can be based on the IEEE1588 protocol. In someimplementations, clock frequency and the absolute timing phase derivedby the RUs should be accurate enough to meet all 3GPP synchronizationrequirements and to ensure that UEs performance is not noticeablyimpacted by any frequency or timing phase error between the RUs and theCU and between the RUs themselves.

To deal with the variable packet delays in an Ethernet network, downlinkair interface framing in the CU and uplink air interface framing in theRUs are advanced by T_(DL) and T_(UL) seconds relative to each other. Inimplementations, these framing advances T_(DL) and T_(UL) have to begreater than a sum of the respective Ethernet network delay between theCU and the RU and the timing phase error between the clocks in the CUand the RU. Since the worst-case clock error is small compared to theworst-case Ethernet delay, it has a lesser effect on the selection ofthe framing advances T_(DL) and T_(UL). When the actual network delaythat a packet experiences exceeds the framing advance, buffer underflowwill occur and physical layer transport packets will be lost. Such aloss can be recovered using retransmissions in HARQ, RLP or TCP layers,but at the expense of reduced transmission efficiency. Therefore, it isimportant that such underflow occurs rarely, and does not impact theuser experience.

One of the features of the present disclosure is its ability to serveUEs via multiple RUs that share the same cell. For example, as describedabove, multiple RUs may be controlled by a CU to define a cell, in whichmultiple UEs may be served. Assigning multiple RUs to the same cell mayreduce the number of baseband modems used in the CU, avoid inter-cellinterference and improve signal strength through macro diversity.Sharing the same cell across multiple RUs may reduce the LTE systemcapacity available to individual users. In some implementations, as longas cell loading remains below 50% of cell capacity, no appreciableperformance degradation will occur.

In order to implement cell sharing in the present disclosure, in someimplementations, the relative carrier frequencies of RUs in the samecell should be frequency synchronized in a way that is tighter than thefrequency accuracy required from individual RUs. In someimplementations, without such tight differential synchronization, theeffective downlink channel seen by the UE may become time-varying in amanner similar to what happens when there is mobility and as a resultthe performance may degrade. Channel variations caused by mobility or bydifferential carrier frequency offset between RUs result in a mismatchbetween the channel measured using the reference signals and the channelactually experienced when demodulating the LTE OrthogonalFrequency-Division Multiplexing (OFDM) symbol.

The tight differential carrier frequency synchronization of RUs asdescribed above will also be required between RUs that belong todifferent cells but use Rel. 11 downlink Coordinated Multipoint (Rel. 11CoMP or simply “CoMP”). In CoMP, at a cell-edge, typically, downlinksignals from two or more RUs that may belong to different cells could bereceived at a UE while the UE's uplink transmissions could also bereceived by these various RUs. If the downlink transmissions to a givenUE can be coordinated, downlink performance can be enhanced. Likewise,if uplink transmissions can be scheduled in a coordinated manner, uplinkperformance can be enhanced. CoMP addresses issues such as interferencemitigation and coordinated bit transmissions over the air interface.

When such tight synchronization cannot be maintained, downlink physicallayer CoMP performance may degrade, potential CoMP gains may be reducedor lost or could even turn negative. Downlink CoMP is a part of thepresent disclosure, but tight differential synchronization requirementsfor some implementations of CoMP are not unique to the presentdisclosure and also apply to other LTE systems that use downlink CoMP.

When multiple RUs share the same cell, the timing phase of theirtransmissions also needs to be synchronized. This synchronization canalso facilitate the radio network of this disclosure to combine uplinksignals received by different RUs in the CU. In some implementations,such combinations require that all significant multipath signalsreceived by different antennas fall within a time interval called cyclicprefix. The cyclic prefix corresponds to the first N_(CP) samples in anOFDM symbol that are a replica of the last N_(CP) samples in the samesymbol. The cyclic prefix ensures that the transmitted subcarrier willremain orthogonal in the receiver, as long as the delay spread of thechannel is less than the N_(CP). When multiple RUs share the same celland there is a timing phase offset between the RUs, the sum of thisoffset and the delay spread of the wireless channel can be controlled soas to not exceed the cyclic prefix length. In the LTE standard, thecyclic prefix is around 5 microseconds. Therefore, it is desirable tokeep the timing phase error between RUs much smaller than 5microseconds.

Following a brief overview of the synchronization requirements in thepresent disclosure, we will describe how these requirements areaddressed.

In this regard, synchronization, and the features described hereinrelating thereto, are example implementations. Different implementationsof the present disclosure may employ different synchronization methodsand variations on any and all of the methods described herein. Anyrequirements specified in this disclosure relate to the specific exampleimplementations described herein only, and are not requirements of anymore general methods, apparatus, systems, and computer program productsthat may be claimed.

In an example implementation of the present disclosure, basebandoperations up to the FFT input are performed in the CU and the remainingbaseband operations (FFT, cyclic prefix, etc.) and the radios areimplemented in the RUs. In another example implementation of the presentdisclosure, on the downlink, baseband operations up to the modulation orlayer mapping are implemented in the controller and the remainingbaseband operations are implemented in the RUs. As previously explained,the CU and the RUs are separated by a switched Ethernet network thatcarries data between the CU and the RUs in packets or frames.

1. Synchronization Between the CU and the RUs

In some implementations, there is a VCTCXO crystal oscillator in CU andVCTCXO crystal oscillators in all of the RUs. The VCTCXO in the CU isused to generate clocks required for the baseband processing in the CUand the VCTCXOs in the RUs are used to generate clocks foranalog-digital-analog converters (A/D/As), RF synthesizers, and basebandprocessing performed in the RUs. In some implementations, only the CUhas a GPS receiver or another timing synchronization mechanism that cangenerate a stable frequency-stable and phase-accurate clock referenceand, therefore, there is a need to provide a frequency-stable andphase-accurate clock reference to the VCTCXOs in the RUs using IEEE1588based timing synchronization. As described by the National Institute ofStandards and Technology (NIST), the IEEE 1588 “standard defines aprotocol enabling precise synchronization of clocks in measurement andcontrol systems implemented with technologies such as networkcommunication, local computing and distributed objects. The protocol . .. [is] . . . applicable to systems communicating by local area networkssupporting multicast messaging including but not limited to Ethernet”.The contents of the IEEE 1588-2002 as published in 2002 and as revisedin 2008 are hereby incorporated by reference into this disclosure.

IEEE1588 is a time-stamping protocol, implemented over the UDP/IPprotocol, between a master clock in the CU and slave clocks in the RU.The protocol involves repeated round-trip exchanges between the masterand slave clocks, where each exchange produces a timing update signalthat can be used to construct a timing reference signal in the RU. Themaster clock starts the exchange by sending a time stamp to the slave inthe RU. This time stamp carries the time T1 as measured by the masterclock at the time the time stamp leaves the Ethernet interface on theCU. The slave receives this time stamp when its local clock is at timeT1′. The difference T1′−T1=D_(DL)+Δ is the sum of the unknown one-waytravel delay D_(DL) of the time stamp from the CU to the RU and theunknown clock phase error A between the reference clock in the RU andthe reference clock in the CU. In order to estimate (and cancel) theone-way downlink delay, the slave sends to the CU a second time stamp.This time stamp carries the time T2 as measured by the slave clock atthe time the time stamp leaves the Ethernet interface on the RU. Themaster marks the time T2′ on its local clock when it receives the timestamp on the Ethernet interface on the CU, and sends value T2′ in aresponse message back to the slave. The difference T2′−T2=D_(UL)−Δ isthe sum of the unknown one-way travel delay of the time stamp from theRU to the CU and the unknown clock phase error (−Δ) between thereference clock in the CU and the reference clock in the RU. If theone-way delay in the two directions were the same (i.e., D_(DL)=D_(UL))and the phase of the reference clock in the CU does not drift relativeto the reference clock in the RU during the exchange, the slave canestimate the clock error A by removing the effect of the one-way delaysby computing:

Δ′=[(T1′−T1)−(T2′−T2)]/2.

This clock phase error estimate Δ′ can be used in the RU to produce areference signal that closely tracks the timing reference signal (e.g.,a GPS-derived, 1 Pulse Per Second (1PPS) signal) in the CU.

In some implementations, the one-way delays in the two directions aregenerally not equal, primarily due to asymmetric load-dependent delaysin the switches (propagation and transmission delays are typicallysymmetric). To reduce the effect of such errors, IEEE 1588v2 introducedthe ability for intermediate nodes, such as Ethernet switches, tomeasure the delays that the packets incur inside the node and insertthis part of the delay into the time stamp packets as they traverse thenode. Such 1588v2 support by Ethernet switches will allow the slave toestimate the round-trip delay without the asymmetric load-dependentnetwork delays and produce a much more accurate estimate of the clockoffset to drive the Phase Locked Loop (PLL). However, switches thatsupport IEEE1588 tend to be more expensive and therefore there is a needto develop methods that can reduce or eliminate the effects ofasymmetric network delays.

To the extent the IEEE1588v2 processes can be used to drive the timingphase error to zero, the reference clock in the RU can be perfectlyaligned in phase and frequency with the reference clock in the CU, forexample a GPS 1PPS signal.

In example systems of the present disclosure, the VCTCXO in the CU isused as the master clock to generate the timestamps for the IEEE1588protocol. The RU's VCTCXO is disciplined using the time stamps receivedby the IEEE1588 slave. Intelligent time stamp transmission andprocessing is used in the CU and the RUs to reduce or eliminate jitterintroduced by random asymmetric Ethernet network delays between the CUand the RU. The timing of timestamp generation in the CU and in the RUsis orchestrated to reduce asymmetric delays. Timestamp generation andprocessing may be implemented on a System-on-Chip (SoC) in both the CUand the RU. Hardware-assist is used in this process to reduce thepossibility that random asymmetric delays are introduced into theIEEE1588 processing.

If the time stamps are sent by the CUs and RUs in an uncoordinatedmanner, they may experience different delays on the uplink and downlinkbecause of different levels of contention they encounter in the twodirections. For example, if multiple RUs respond to a time stamp sent bythe CU at about the same time, the uplink time stamps may experiencesignificantly longer delays than the time stamps sent on the downlink.Contention between time stamps and IQ data may also contribute toincreased latency and such latency may be different in the twodirections.

Two metrics that can be used to assess the accuracy of the IEEE1588timing synchronization method are the mean value and the variance of theclock error estimate Δ′:

$\begin{matrix}{{ {{E\{ \Delta^{\prime} \}} = {{E\{ ( {{T1^{\prime}} - {T1}} ) \}} - {E\{ ( {{T2^{\prime}} - {T2}} ) \}}}} \rbrack/} = {\lbrack {{E\{ {D_{DL} + \Delta} \}} - {E\{ {D_{UL} - \Delta} \}}} \rbrack/2}} \\{= {\Delta + {E\{ {D_{UL} - D_{UL}} \}/2}}}\end{matrix},$

Where E{ } refers to statistical expectation or mean value of itsargument. In other words, the mean of the timing estimate Δ′ has a fixedbias which corresponds to the average delay difference between theuplink and the downlink, divided by 2. When the average delays on the DLand UL differ significantly, there could be a significant phase error inthe average timing estimate. The variance of the timing estimate isproportional to the variance of ½ the difference between DL and ULdelays.

E{(Δ′−E{Δ′})²}=variance{(D _(DL) −D _(UL))/2}.

The mean-squared estimation error E{(Δ′−Δ)²} between the estimated clockphase error and the actual clock phase error will be higher than thevariance of Δ′ by the square of the bias:

E{(Δ′−Δ)²})=variance{D _(DL) −D _(UL)/2}+[E{D _(DL) −D _(UL)}/2]².

In some implementations, it is possible for the RU to accuratelydetermine the ratio between the UL and DL delays; e.g., D_(UL)/D_(DL)=a.The RU can then modify the formula for the clock error estimateaccording to:

Δ′=[a(T1′−T1)−(T2′−T2)]/(1+a).

To the extent the parameter “a” can be determined exactly, a perfectestimate of the clock error can be obtained with no bias; i.e., E{Δ′}=Δand variance {Δ′}=0. In some implementations, it is difficult to knowthe uplink and downlink delays exactly in a consistent manner. Sometimesit may be possible to determine a functional relationship between theuplink and downlink delays on average. For example, if there is a knownfunctional relationship between the average delays D₁=E{D_(DL)} andD₂=E{D_(UL)}, then it is possible to reduce or even remove the bias termE{D_(DL)−D_(UL)}/2. For example, if D₂=a D₁+b, in other words theaverage delay in the UL is a known linear function of the average delayon the DL, then we can reduce or remove the bias by using a modifiedtiming estimate given by the following:

Δ′=[a(T1′−T1)+b−(T2′−T2)]/(1+a).

In this case, it can be shown that E{Δ′}=Δ, which is the correctestimate with no bias. It can be observed that in the special case wherea=1 and b=0, this reduces to the case where the average delays on the ULand DL are the same and the timing estimate reduces to the standard 1588timing estimation formula.

The variance of the timing phase estimate is now reduced to:

E{(Δ′−E{Δ′})² }=E{(Δ′−Δ′)²}=variance{aD _(DL) +b−D _(UL)/(1+a)}.

Another method for reducing the mean-squared timing phase error is tominimize both the mean and the variance of the average delaydifferential between the uplink and the downlink by controlling thetransmission of the time stamps relative to each other and relative tothe IQ data transmissions between the CU and the RU so as to avoidcontention in the switches. Next we describe an example method that cansignificantly reduce the downlink and uplink delays.

In this method, we let the CU and each RU execute multiple time stampexchanges during a given time interval A, e.g., 1 second. For example,the CU and the RU may execute 10 time stamp exchanges during a 1 secondinterval, where each time stamp exchange uses 3 IEEE1588 messagetransmissions as described earlier. In some implementations, referringto FIG. 18 , the CU sends 2502 its time stamp in the beginning of theOFDM symbol interval. It then waits 2504 for some pre-configured periodof time before transmitting 2506 its IQ data to allow time for the timestamp to travel through the switches. The time stamp transmissions areassociated with of the highest priority. If a time stamp encounterscontention from IQ data in the switches, it will at most wait for thetransmission of the IQ data whose transmission has already started. Uponreceiving 2508 the time stamp, the RU initiates the transmission of itsown time stamp at randomly chosen intervals later. In someimplementations, upon receiving the time stamp from the CU, the RU maywait 2510 a pre-configured time interval before transmitting 2512 thetime stamp. The pre-configured time interval may also depend on the timewhen the RUs own uplink IQ data transmission is completed. The CU, uponreceiving 2514 the RU's time stamp, marks 2516 the time on its localclock and sends this measured time to the RU in another IEEE1588message. The RU upon receiving 2520 this message calculates 2522 anestimate of the clock phase error (or equivalently, a clock offset), butdoes not make any adjustment to its clock. In some implementations, theCU and the RU repeat the above exchange multiple times during the timeinterval A. At the end of the time interval, the RU compares 2524 theclock offsets and updates 2524 its clock based on the measurement thatcorresponds to the lowest clock offset.

In some implementations, the RU may compare the clock offset to athreshold value. If the clock offset exceeds the threshold value inmagnitude, the RU does not update its clock during an interval A. Inaddition to computing the estimates for the clock offset, the RU cancompute the round trip delay as

D _(DL) +D _(UL)=[(T1′−T1)+(T2′−T2)].

A round trip delay may indicate that the IEEE1588 exchange hascontention, and that that the associated clock offset is inaccurate, andtherefore, should not be used.

The CU also implements similar IEEE1588 exchanges with other RUs. Insome implementations the CU may implement the IEEE1588 exchanges withdifferent RUs in a non-overlapping fashion, so as to minimize contentionin uplink time stamp transmissions. In some implementations, only oneIEEE1588 exchange may be used for each RU during the time interval A.

If there are multiple controllers at the site sending traffic to thesame output port of a switch, these transmissions may also createcontention and increase latency. One way such contention may be avoidedis to use a single controller to act as the master for all DLtransmissions. In other words, all traffic may be routed through themaster controller. Alternatively, a single controller may assume themaster role only for the IEEE1588 operation. In this case, only themaster controller will send time stamps to the RUs.

If the RUs and the controller support other traffic, such as Wi-Fitraffic, the transmission of the other traffic may also be timed toavoid contention in the switches. For example, additional Ethernet linksmay be used to avoid direct contention between such other traffic andthe latency-sensitive IQ data and IEEE1588 time stamp traffic.

In some implementations, traffic associated with different controllersand other traffic, such as WiFi, can be segregated, e.g., strictlysegregated, by assigning them to different VLANs and using dedicatedEthernet links and ports for the radio network to avoid contention.

Ethernet QoS capabilities can be implemented to improve the performanceof the above methods. Using priority levels defined in the 802.1pstandard, time stamp transmissions can be given higher priority tominimize delays in switches that may be caused by IQ data transmissions.

Next a description is provided of how uplink and downlink subframestransmitted across the switched Ethernet network should be aligned.

2. Frame Advance

Aligning the downlink and uplink transmissions at the antennas in astandalone eNodeB can create a slight misalignment in the eNodeBbaseband processor. But, since the delay between the antennas and thebaseband processor is relatively small, this has little, if any, impacton the system performance. However, the delay in the present disclosurebetween baseband processing in the CU and the antennas near the RUs canbe significantly higher than in a standalone eNodeB because of thedelays introduced by the Ethernet network between the CU and the RUs. Insome cases, the fixed delay between the CU and the RU can be in theorder of 200-300 μs, or 3-4 OFDM symbol intervals. To compensate forthis delay, one may advance the downlink subframe timing in the CU by apre-determined amount of T_(DL) seconds, where T_(DL) is on the order of200-300 μs in some implementations. If the uplink (UL) and downlink (DL)frames are aligned at the RU antenna then, as described below, an offsetwill occur between the UL and DL subframes in the baseband modem of theCU. One timing synchronization requirement in LTE is related to therelative timing phase of uplink transmissions from different UEs. Thisrequirement, called the Uplink Timing Advance, is also implemented inthe present disclosure. In Uplink Timing Advance, the UEs advance thetiming phase of their uplink transmissions relative to received downlinktransmissions based on commands received from the eNodeB. A standardeNodeB determines the timing advance commands to align the start of thereceived n'th uplink subframe with the start of its own downlinktransmission of the n'th subframe at the antennas. If the UE's timingadvance is set equal to the round-trip delay between the UE and theeNodeB antennas, the uplink signals from different UEs will bephase-aligned at the eNodeB antennas.

Accordingly, in the present disclosure, uplink signals from differentUEs are timing-phase aligned at the receive antennas of the RU such thatthese transmissions are all received within the cyclic prefix asexplained earlier. One can then choose the timing advance (TA) accordingto TA=t_(RT), where tar is the mean round-trip delay between the UE andthe nearby RU antennas. This would automatically align the DL subframeboundaries, which are phase-aligned with GPS 1PPS, with UL subframeboundaries at the RU antenna as shown in FIG. 12 . However, the DL andUL subframe boundaries at the CU are now offset with respect to eachother by T_(RT)=T_(DL)+T_(UL), where T_(DL) and T_(UL) are the assumedfixed downlink and uplink frame timing advance between the CU and theRU, respectively. In summary, in the RU TX (transmit) antenna, thetransmission of the n'th DL subframe starts at the same time as thereception of the n'th UL subframe, but in the CU the reception of then'th UL frame occurs T_(RT) seconds later than the start of thetransmission of the n'th DL subframe. A drawback of this approach isthat the HARQ processing time in the CU may be reduced by T_(RT)seconds, which can be as high as 500 ρs. In implementations where thereis no delay between the RU and the CU, the controller has 3 ms availableto process the signals received on the uplink and start thecorresponding transmission on the downlink. Therefore, this couldrepresent a reduction of 17% in processing time available in the CU.

Consider the downlink HARQ operation of FIG. 13 , where the CU sendsPDSCH data in DL subframe N, which is received by the UE afterT_(DL)+t_(DL) seconds. The UE sends an ACK/NAK message in uplinksubframe N+4. If the timing advance TA=t_(RT), as would be the case in aclassic eNodeB, then from the end of DL subframe N to the beginning ofUL subframe N+4, the UE has 3−TA=3−t_(RT) ms to demodulate the DLsubframe N, determine the ACK/NAK and construct the ACK/NAK message.From the time it receives the UL subframe N+4 carrying the ACK/NAK, theCU can have until the beginning of DL subframe N+8 to schedule aretransmission. When TA=t_(RT), then from the end of the N+4'th ULsubframe to the beginning of the N+8'th DL subframe, the CU will haveonly 3−T_(RT) ms available to start a retransmission. In other words,the available processing time in the CU is reduced by the round-tripdelay between the CU and the antenna. In some implementations, the CUmay delay the retransmission by taking advantage of so-called adaptivefeature of the DL HARQ, though in some circumstances this may reduce theoverall throughput. A similar reduction in available processing timealso occurs in uplink HARQ, where the CU has 3−(T_(DL)+T_(UL))processing time between receiving an uplink transmission and sending anACK/NAK on the downlink.

A method that can address the above issue is to increase the uplinktiming advance TA by T_(RT) for all the UEs. This does not affect theuplink timing phase alignment among UEs at the RU, since the timingadvance is increased by the same amount for all the UEs. As explainedabove, increasing the timing advance reduces the HARQ processing time inthe UE, but since all the UEs are designed to handle a maximum timingadvance of 667 ρs in some implementations, there should not be anyproblems as long as the timing advance is kept below this limit. Thesubframe alignment in this case is illustrated in FIG. 14 .

As required, the DL subframes are phase aligned with GPS 1PPS at the TXantennas, but the UL subframes at the RX antennas are now offset byT_(RT) seconds relative to GPS 1PPS. In other words, the RU will startprocessing UL subframe N T_(RT) seconds before it starts processing DLsubframe N.

The revised HARQ timing for both downlink and the uplink are illustratedin FIGS. 15 and 16 . In the examples shown in both figures, theprocessing time in the CU remains constant at 3 ms, whereas theprocessing time in the UE is reduced to 3−t_(RT)−T_(RT) ms, but is stillwithin the bounds of UE's capabilities. It is possible to choose thetiming advance to be anywhere between t_(RT) and t_(RT)+T_(RT).

When the UE applies a large timing advance TA, the preambleconfiguration for the Physical Random Access Channel (PRACH) needs to beselected accordingly to prevent the PRACH preamble transmission insubframe N from interfering with Physical Uplink Shared Channel(PUSCH)/Physical Uplink Control CHannel (PUCCH) transmissions in thenext subframe N+1. The guard interval GI of the preamble should begreater than the timing advance TA or alternatively, the eNodeB shouldnot schedule any PUSCH transmissions in subframe N+1 in the resourceblocks assigned to PRACH in subframe N. PRACH preamble formats 1 and 3support a TA of up to 0.52 and 0.72 ms, but use 2 and 3 subframes,respectively. PRACH preamble formats 0 and 2 only support a TA of up to0.1 and 0.2 ms, using 1 and 2 subframes, respectively. PRACH preambleformat 1 is suitable for the present disclosure if TA can be kept below0.5 ms. Alternatively it is possible to use a format 0 and not toschedule any PUSCH transmission in the PRACH RBs in the uplink subframeimmediately following the PRACH transmission.

In the CU the PRACH packets sent by the RU are stored in a PRACH buffer,separate from the UL buffer, and are processed as quickly as possible.The CU examines the 839-point energy sequence, determines whether apreamble is present and, if so, estimates the cyclic shift that wastransmitted by the UE, and prepares the PRACH response message. Whilethere is no strict timing requirement for the CU to send a PRACHresponse, this should be done as quickly as possible in order to improvethe PRACH response time. Based on FIG. 16 , the CU can send the PRACHresponse in subframe N+3.

In some implementations, the TA in the UE may be kept low, for exampleas low as the round-trip airlink delay, and one may accept the resultingreduced processing time in the controller. This may allow the radionetwork to use the Format 0 PRACH preamble, which uses less airlinkresources for PRACH, or not have any restrictions in scheduling due topotential collisions with PRACH, as described earlier.

3. Frame Alignment for TD-LTE

In frame advance in FDD-LTE. In TD-LTE, the frame structure is designedsuch that uplink and downlink transmissions do not overlap at the RU andUE receive antennas. A special subframe can be used to transition fromDL to UL transmission, as shown in FIG. 19 . The special subframe startswith a few OFDM symbols of DL transmission, followed a silence gapinterval GP that lasts a few OFDM symbols and ends with 1 or 2 OFDMsymbols of UL transmission. The UL transmission in the special subframecan only carry SRS or PRACH (which needs two OFDM symbols). LTE standardsupports 9 different configurations for the special subframe as shown inthe Table 2.

TABLE 2 Subframe Configurations Supported by LTE. Special SubframeConfiguration DL P L Total 0 3 0 14 1 9 14 2 11 14 3 11 14 4 12 14 5 314 6 9 14 7 10 14 8 11 14

As in FDD, the UE advances the UL frame timing relative to the receivedDL timing by TA seconds. This aligns transmissions by different UEs atthe RU antennas. In TD-LTE, the maximum expected value of TA determinesthe gap interval GP. In order to avoid simultaneous DL and ULtransmissions at the UE or RU receive antennas, GP is selected such thatGP≥TA≥t_(RT), where t_(RT) represents the round-trip airlink propagationdelay between the UE and RU antennas.

As shown in FIG. 19 , if GP<TA, the UE's UL transmission at the end ofthe special subframe will interfere with the reception of the DLtransmission in the beginning of the same special subframe. IfGP<TA−t_(RT), then the RUs DL transmission in the beginning of thespecial subframe will cause interference into the RUs reception of theUL transmission at the end of the special subframe. If TA<t_(RT), thenthe RUs DL transmission immediately following an UL-to-DL transitionwill interfere with the RUs reception of the UE's last UL subframetransmission before the UL-to-DL transition.

In some implementations, it is possible for the controller to choose TAto align DL and UL transmissions at the controller as in FDD in order topreserve the 3 ms processing time. The special subframe configurations 0or 5 can be used, which support a GP value (9 or 10 OFDM symbols) thatis large enough to avoid the UL-DL interference described above.Sometimes, the large value of GP can cause inefficiency on DLtransmissions.

In some implementations, a shorter TA value may be used for TD-LTE. InTD-LTE, the HARQ timing is different from that in FDD and depends on thespecific TDD frame configuration. Table 3 shows the minimum HARQ timingrequirements for the 9 different frame configurations that are supportedin the standard. The frame configuration is sent by the controller in aSIB message.

TABLE 3 Minimum HARQ Timing Requirements for 9 Different FrameConfigurations Subframe # 0 1 2 3 4 5 6 7 8 9 0 D S U U U D S U U UACK/NAK 4 6 4 7 6 4 6 4 7 6 Re-Transmission 6 4 6 4 4 6 4 6 4 4 TotalTime 10  10  10  11  10  10  10  10  11  10  1 D S U U D D S U U DACK/NAK 7 6 4 6 4 7 6 4 6 4 Re-Transmission 4 4 6 4 6 4 4 6 4 6 TotalTime 11  10  10  10  10  11  10  10  10  10  2 D S U D D D S U D DACK/NAK 7 6 6 4 8 7 6 6 4 8 Re-Transmission 4 4 4 4 4 4 4 4 4 4 TotalTime 11  10  10  8 12  11  10  10  8 12  3 D S U U U D D D D D ACK/NAK 411  6 6 6 7 6 6 5 5 Re-Transmission 4 4 4 4 4 4 4 4 4 4 Total Time 8 15 10  10  10  11  10  10  9 9 4 D S U U D D D D D D ACK/NAK 12  11  6 6 87 7 6 5 4 Re-Transmission 4 4 4 4 4 4 4 4 4 4 Total Time 16  15  10  10 12  11  11  10  9 8 5 D S U D D D D D D D ACK/NAK 12  11  6 9 8 7 6 5 413  Re-Transmission 4 4 4 4 4 4 4 4 4 4 Total Time 16  15  10  13  12 11  10  9 8 17  6 D S U U U D S U U D ACK/NAK 7 7 4 6 6 7 7 4 7 5Re-Transmission 8 7 6 4 4 7 6 6 7 5 Total Time 15  14  10  10  10  14 13  10  14  10 

For each frame configuration, Table 3 shows the DL (D), UL (U) andSpecial (S) subframes in a radio frame. Configurations 3-5 support asingle DL-UL transition and the other configurations support two DL-ULtransitions within a 10 ms radio frame. For each frame configuration,Table 3 also shows the number of subframes between the transmission ofthe shared channel data and the transmission of ACK/NAK by the receivingnode. In DL HARQ, the ACK/NAK time varies between 4 and 13 subframes.Sometimes the UE will have 3-TA ms processing time available, same as inFDD. In UL HARQ the ACK/NAK time varies between 4 and 7 subframes. WhenDL capacity requirements are higher than that on the UL, configurations2-5 can be used for in-building systems. In these configurations, theACK/NAK time is fixed at 6 subframes, 2 subframes longer than in FDD.This gives the controller 5−T_(R)L+t_(RT) seconds of processing time. IfTA is minimized by setting it equal to the round-trip airlink delay,i.e., TA=t_(RT), then the available processing time is 5−T_(RT). If TAis chosen to also compensate for the controller-RU round-trip delayT_(RT), i.e., TA=T_(RT)+t_(RT), then the available time is 5 subframes,which is 2 subframes longer than in FDD.

Table 3 also shows the retransmission time. It can be seen that the DLretransmission time varies between 4 and 8 subframes, but forconfigurations 3-5 it is always equal to 4, the same as in FDD. Theavailable processing time in the controller increases from 3−T_(R) to 3ms as TA is increased from t_(RT) to t_(RT)+T_(RT). This is the sametrade-off as in FDD. In the UL the retransmission time varies between 4and 7 subframes. In the worst-case of 4 subframes, the availableprocessing time in the UE is the same as in FDD.

In TD-LTE PRACH opportunities are allowed in UL subframes. PRACHopportunities may also be created in special subframes when at least 2OFDM symbols are assigned to PRACH (special subframe configurations5-8). But in this case, the available silence interval is 288 samples(at 20 MHz), or 9.375 ns, which limits the round-trip airlinkpropagation delay to 9.375 ns, or about 1.4 km. This shows that inin-building networks, special subframes can be used for PRACH when UL/DLframes are aligned at the RUs and reduced processing time that may beavailable in the controller in certain configurations is accepted. Theuse of PRACH in normal UL subframes is the same as in FDD, except inTD-LTE multiple PRACH opportunities can be supported in a singlesubframe.

Other Embodiments

Although various assumptions are made for the purpose of discussion, theimplementations of the systems and methods described in this disclosureare not limited by these assumptions. Instead, the discussions based onthese assumptions can be readily generalized to other situations. Thenumbers of RUs in each cell, the numbers of antennas for each RU, andthe numbers of cells in a network can vary, e.g., based on the networkdemands.

What is claimed is:
 1. A communication system comprising: remote unitsto wirelessly exchange radio frequency (RF) signals with mobile devices,each RF signal comprising information that is destined for, ororiginating from, at least one of the mobile devices; and a controllercommunicatively coupled to the remote units; wherein the remote units(RUs) and the controller communicate baseband data corresponding to theinformation across an intermediate network; wherein the remote unitseach implement at least some physical layer processing for an airinterface used to wirelessly communicate with the mobile devices; andwherein two or more of the remote units transmit a downlink referencesignal to a mobile device of the mobile devices.
 2. The communicationsystem of claim 1, wherein each of the two or more of the remote unitsbelongs to a same cell.
 3. The communication system of claim 2, whereineach of the remote units in the cell comprise one or more antennas, theone or more antennas being associated with a common cell identifier. 4.The communication system of claim 1, wherein a single broadcast channelphysical broadcast channel (PBCH) is used in the cell.
 5. Thecommunication system of claim 1, wherein the downlink reference signalis a Channel State Information Reference Signal (CSI-RS).
 6. Thecommunication system of claim 5, wherein a number of RU clusters aredefined, each RU cluster comprising at least one RU and assigned to adifferent CSI-RS resource with a distinct Channel State Information(CSI) scrambling ID.
 7. The communication system of claim 6, wherein thecontroller configures the mobile device to monitor multiple CSI-RSresources.
 8. The communication system of claim 1, wherein each of theremote units comprises one or more physical antennas, wherein eachphysical antenna corresponds to a virtual antenna port, wherein at leastsome of the physical antennas across different remote units in an RUcluster are assigned to a same virtual antenna port.
 9. Thecommunication system of claim 8, wherein all physical antennas assignedto the same virtual antenna port are configured to transmit a commonsignal.
 10. The communication system of claim 1, wherein theintermediate network is an Ethernet network.
 11. A method comprising:wirelessly exchanging, by remote units, radio frequency (RF) signalswith mobile devices, each RF signal comprising information that isdestined for, or received from, at least one of the mobile devices;communicating baseband data corresponding to the information between acontroller and the remote units (RUs) across an intermediate network;performing, at the remote units, at least some physical layer processingfor an air interface used to wirelessly communicate with the mobiledevices; and wherein two or more of the remote units transmit a downlinkreference signal to a mobile device of the mobile devices.
 12. Themethod of claim 11, wherein each of the two or more of the remote unitsbelongs to a same cell.
 13. The method of claim 12, wherein each of theremote units in the cell comprise one or more antennas, the one or moreantennas being associated with a common cell identifier.
 14. The methodof claim 11, wherein a single broadcast channel physical broadcastchannel (PBCH) is used in the cell.
 15. The method of claim 11, whereinthe downlink reference signal is a Channel State Information ReferenceSignal (CSI-RS).
 16. The method of claim 15, wherein a number of RUclusters are defined, each RU cluster comprising at least one RU andassigned to a different CSI-RS resource with a distinct Channel StateInformation (CSI) scrambling ID.
 17. The method of claim 16, wherein thecontroller configures the mobile device to monitor multiple CSI-RSresources.
 18. The method of claim 11, wherein each of the remote unitscomprises one or more physical antennas, wherein each physical antennacorresponds to a virtual antenna port, wherein at least some of thephysical antennas across different remote units in an RU cluster areassigned to a same virtual antenna port.
 19. The method of claim 18,wherein all physical antennas assigned to the same virtual antenna portare configured to transmit a common signal.
 20. The method of claim 11,wherein the intermediate network is an Ethernet network.